What fallacy is it when someone says "this is true/it happened, therefore there are good reasons for it"?
For example:
We drive on the left (or right, depending on country) side of the road, therefore there must be a good reason for it.
Assume, for the sake of argument, that both sides are an equally good choice.
The fallacy part is the claim that reasons exist when in reality they don't. The original event/fact is the result of an arbitrary decision. There were no "good reasons".
Taking into account the fact that the phrase "good reason" has a normative import, the reasoning could be reconstructed in this way:
(1) It is the case that X.
(2) If it is the case that X, then it is normal/correct/practically necessary that X.
(3) Then, it is normal/correct/practically necessary that X.
Premise (2) seems to be analogous to the "is/ought" confusion.
https://www.txstate.edu/philosophy/resources/fallacy-definitions/Is-ought.html
Is Ought
The is-ought fallacy occurs when the assumption is made that because things are a certain way, they should be that way. It can also consist of the assumption that because something is not now occurring, this means it should not occur. In effect, this fallacy asserts that the status quo should be maintained simply for its own sake. It seeks to make a value of a fact or to derive a moral imperative from the description of a state of affairs.
Note: by "practically necessary", I mean necessary in the order of practice (i.e. what one has to do or to comply with).
The error is to leap from "everything happens for a cause/reason" to "everything happens for a good (or desirable) cause/reason".
Example:
The error is in Step 3.
This error could arise from (but is not identical to) the Panglossian or "best of all worlds" fallacy. Or more simply from an excess of optimism about the world.
What fallacy is it when someone says "this is true/it happened, therefore there are good reasons for it"?
There is no fallacy described here. The argument uses the Principle of Sufficient Reason.
The Principle of Sufficient Reason is simply stated: “For every fact F, there must be an explanation why F is the case” (Melamed and Lin 2016, §1). The principle is most closely associated with Gottfried Wilhelm Leibniz (Melamed and Lin 2016, §3), although forms of the principle first appeared in antiquity (Melamed and Lin 2016, §4). Leibniz joined the principles of noncontradiction and sufficient reason:
Our reasonings are grounded upon two great principles, that of contradiction, in virtue of which we judge false that which involves a contradiction, and true that which is opposed or contradictory to the false;
And that of sufficient reason, in virtue of which we hold that there can be no fact real or existing, no statement true, unless there be a sufficient reason, why it should be so and not otherwise, although these reasons usually cannot be known by us. (Leibniz, cites omitted; Melamed and Lin 2016, §3)
Assume there has been an observation, “Event P and Event Q”. The relation between Events P and Q might be that of cause and effect, or there might be no relation at all beyond randomness. But whatever that relation might be, there is a reason why it is so and not otherwise.
Hume denies the validity of the Principle of Sufficient Reason. He considers several arguments in its support, including those from Thomas Hobbes and John Locke (Hume, THN, I, 3, 3). Although this principle is said to be “impossible for men in their hearts really to doubt”, Hume finds “no mark of any such intuitive certainty” (Hume, THN, I, 3, 3). Hume would likely agree that the argument described in the question is fallacious, as the Principle of Sufficient Reason is wrong.
My own view is that the validity of the Principle is the common sense of the matter.
Sources: Hume, David. A treatise of human nature [“Hume, THN”] http://www.gutenberg.org/files/4705/4705-h/4705-h.htm#link2H_4_0023
Melamed, Yitzhak and Lin, Martin, "Principle of Sufficient Reason", The Stanford Encyclopedia of Philosophy (Spring 2016 Edition), ed. Edward N. Zalta. http://plato.stanford.edu/entries/sufficient-reason/.
"Everything happens for a reason" is usually used in a different context, as a way to cope with senseless tragedies caused by random disasters. It is a case of pathetic fallacy, ascribing human reasons to inanimate things.
"The statement that everything happens for a reason does not explain away randomness, and in fact it dismisses important truths about the senselessness of some events. Sometimes the reasons for things lie in unthinking, unemotional nature and have nothing to do with human truth. The most egregious instances of this phrase occur where people use it to dismiss senseless tragedies. It is true that tsunamis, hurricanes, earthquakes, and floods happen for natural, earthly reasons, but notwithstanding the human need for comfort, there is no way to reconcile the random loss of life with morality."
The OP example is an extension of this tendency to human affairs: the blissful belief that humans, at least, do things "for good reasons". And it is a lazy variant of it, the "good reasons" are not even manufactured, just asserted as dogma. But the sentiment had high currency in classical philosophy. Socrates and scholastics believed that free decisions are bound to be rational, Leibniz declared this world to be the best of possible worlds, with God intended reasons behind all events, Hegel held that in history "the real is the rational", and so on. The idea was even coined into the principle of sufficient reason, in Leibniz's rendition:
"Our reasonings are based on two great principles, that of contradiction... and that of sufficient reason, by virtue of which we consider that we can find no true or existent fact, no true assertion, without there being a sufficient reason why it is thus and not otherwise, although most of the time these reasons cannot be known to us."
The idea that human/cultural choices are rational choices is sometimes called the rational fallacy, or the fallacy of rationalization. Fake rationalization is also a well known behavioral mechanism in psychology. An extreme expression of it is apophenia, "unmotivated seeing of connections". Taleb in Black Swan called the narrative fallacy this tendency of weaving explanations into facts that lack them, in denial of irrationality and randomness of life:
"The narrative fallacy addresses our limited ability to look at sequences of facts without weaving an explanation into them, or, equivalently, forcing a logical link, an arrow of relationship upon them. Explanations bind facts together. They make them all the more easily remembered; they help them make more sense. Where this propensity can go wrong is when it increases our impression of understanding."
The idea spread also into the classical economics, where Adam Smith and others assumed that free market agents make rational decisions. Keynes denied this assumption in his conception of “animal spirits”, a combination of instincts, biases and emotions, that make people swing between irrational exuberance and pessimistic despair, and argued for government action (cyclic stabilizers) to counter this irrationality.
tl;dr– Sounds like the basic premise behind conservativism (as opposed to liberalism). In general, both conservatives and liberals favor intelligent consideration when able; but, when it's unclear if a tradition has due motivation, more conservative positions weight the tradition's possible wisdom more heavily while more liberal positions weight the tradition's possible wisdom less heavily.
Consider the 5-monkeys thought experiment (which apparently wasn't actually conducted):
Four monkeys are put into an enclosure with a bunch of bananas hanging around. But whenever anyone goes near the bananas, they're hosed down with cold water.
New monkeys are introduced into the enclosure over time. Whenever a new monkey goes near the bananas, the others attack it.
Monkeys are also removed over time. Eventually, the enclosure contains no monkeys who were hosed down, but they all still observe the taboo against going near the bananas, even after the hose is removed.
Do real monkeys behave this way? Unclear. Still, the thought experiment's interesting: how should monkeys behave in this situation? What about humans?
There're two extreme positions to take:
Pure conservativism: We should respect the traditional avoidance of the bananas, which probably had good reason even if we're not aware of it.
Pure liberalism: If the bananas look yummy, let's get them! Traditional superstitions don't matter.
Obviously, neither approach is perfect. Conservatives will unnecessarily avoid the bananas even after the cold-water hose is removed, while liberals will keep trying for the bananas and getting hosed with cold water.
Obviously, using intelligent consideration of historical rationality is better. For example, if the monkeys could record their reasons for avoiding the bananas, then they could weigh their desire to try for the bananas vs. their desire to not get hosed to inform future behavior.
However it's not possible to fully record all history and reasoning, so intelligent-consideration is a limited technique; eventually, folks have to decide to what degree traditional reasoning ought to be observed despite not knowing a reason for it.
Point being, it's conservativism to observe traditional biases, while it's liberalism to disregard them. While imperfect, conservatism isn't really a fallacy because liberalism isn't perfect, either; limited information about history forces guesswork.
Intelligent consideration of history is always superior to either conservativism or liberalism.
For example, if we can look up the historical reasons for driving on the right/left side-of-the-road, then we can intelligently consider them. But, if we don't have access to that information, then:
the conservative tactic would be to favor the traditional approach, as it may've been based on a good reason;
the liberal tactic would be to disregard the traditional approach, because we don't know if there was a good reason for it, or even if there was, if such a reason continues to be meaningful today.
That said, someone who absurdly insists on respecting/contradicting tradition when intelligent-consideration is viable is just being silly. We might regard such silly positions as straw-conservativism and straw-liberalism.
It could be a causal fallacy, for example one of the type "cum hoc, ergo propter hoc". That is to say: Correlation does not imply causation.
EDIT: Not everything that happens, happens for good reason. So if you don't know the cause, you can't classify it as unique or even good. (fallatia ad ignorantiam)
Formally, the argument is of the form:
We live in the best of all possible worlds
If we live in the best of all possible worlds, then if something happens then there is a good reason for it.
Something happened
Therefore there is a good reason for it
Or, symbolically:
A
A-->(B-->C)
B
∴ C
This argument is, in these terms, valid. If 'We live in the best of all possible worlds' (or similar) and the corollary premise regarding good reasons are not premises, then the argument is a nonsequitur, which is to say it commits the very general catch-all formal fallacy of possessing a conclusion that does not follow from its premises.
Informal fallacies aren't absolute logical truths but rather ideas about what sorts of arguments are bad. Usually, informal fallacies have an implied premise which those believing the reasoning to be fallacious categorically reject. For example, an ad hominem argument might carry the implicit premise 'people with stupid faces can't be right'. If we accept that premise, the argument "You are not right because you are a person and your face is stupid" is a totally valid argument from a formal logic standpoint. Nevertheless, because it is sufficiently universally agreed that 'people with stupid faces can't be right' is false we can categorically dismiss all arguments that rely on that premise.
The problem with your category of fallacious argument is that a lot of people, including many notable and influential philosophers and scholars throughout the ages, genuinely affirm the position that everything happens for a reason. For example, the vast majority of Christian, Jewish, and Islamic thought-- St. Augustine, Al-Ghazali, Avicenna, St. Thomas Aquinas, Philo of Alexandria, to name a few-- for the last thousand years strongly affirms the idea that the universe is fundamentally ordered by God and consequently everything which exists must have some good reason that it has been permitted to exist. Consequently, there isn't a universally recognized fallacy here, since the falsity is very much still under dispute. There are a lot of people who believe this position is false, as well, and that has given rise to terms like e.g. Just-World fallacy, but these terms are not very well established and do not enjoy broad recognition the way e.g. ad hominem or petitio principii do.
I don't know the name of the fallacy. But there are several problems with it.
Someone has an intention. That some specific consequence shall occur.
Based on 1) and some information together with some prediction according to some judgement something was done.
The consequences of 2) together with any other things which can affect it occurred.
The reasoning is good assuming both that the intentions in 1 are good, that the choice of action in 2 is good and that the consequence in 3 also was good (as intended).
But the intention can be bad.
Or it can be good, but the judgement of what to do was crappy.
Or even the judgement was good, but due to lack of or faulty information the prediction was bad.
Or both intention, judgement and prediction was good but some unforeseen action from a third party screwed up the consequence.
Sometimes nothing was done at all and everything that caused the consequence was due to third party.
