In the The Letters of J.R.R. Tolkien, Tolkien actually expresses in multiple instances that for the world of The Lord of the Rings and The Hobbit he does not believe in the 'absolutes' of either evil or good.

Specifically, he doesn't believe in absolute evil:

In my story I do not deal in Absolute Evil. I do not think there is such a thing, since that is Zero. I do not think that at any rate any 'rational being' is wholly evil. Satan fell. In my myth Morgoth fell before Creation of the physical world. In my story Sauron represents as near an approach to the wholly evil will as is possible. He had gone the way of all tyrants: beginning well, at least on the level that while desiring to order all things according to his own wisdom he still at first considered the (economic) well-being of other inhabitants of the Earth. But he went further than human tyrants in pride and the lust for domination, being in origin an immortal (angelic) spirit.

and he doesn't believe in absolute good, either:

Some reviewers have called the whole thing simple-minded, just a plain fight between Good and Evil, with all the good just good, and the bad just bad.

...

[Besides], in this 'mythology' all the 'angelic' powers concerned with this world were capable of many degrees of error and failing, between the absolute Satanic rebellion and evil of Morgoth and his satellite Sauron, and the fainéance of some of the other higher powers or 'gods'. The 'wizards' were not exempt. Indeed, being incarnate, they were more likely to stray, or err. Gandalf alone fully passes the tests, on a moral plane anyway (he makes mistakes of judgement). Since in the view of this tale and mythology, Power, when it dominates or seeks to dominate other wills and minds (except by the assent of their reason) is evil, these 'wizards' were incarnated in the life-forms of Middle-earth, and so suffered the pains both of mind and body.

And as to the 'why', he pens this to C.S. Lewis (also documented in the same book) where he expresses his own personal opinion about what it means to suffer:

It is one of the mysteries of pain that it is, for the sufferer, an opportunity for good, a path of ascent however hard. But it remains an ‘evil’, and it must dismay any conscience to have caused it carelessly, or in excess, let alone wilfully.

In the The Letters of J.R.R. Tolkien, Tolkien actually expresses in multiple instances that for the world of The Lord of the Rings and The Hobbit he does not believe in the 'absolutes' of either evil or good.

Specifically, he doesn't believe in absolute evil (letter 183 to W.H. Auden):

In my story I do not deal in Absolute Evil. I do not think there is such a thing, since that is Zero. I do not think that at any rate any 'rational being' is wholly evil. Satan fell. In my myth Morgoth fell before Creation of the physical world. In my story Sauron represents as near an approach to the wholly evil will as is possible. He had gone the way of all tyrants: beginning well, at least on the level that while desiring to order all things according to his own wisdom he still at first considered the (economic) well-being of other inhabitants of the Earth. But he went further than human tyrants in pride and the lust for domination, being in origin an immortal (angelic) spirit.

and he doesn't believe in absolute good, either (letter 154 to Naomi Mitchison):

Some reviewers have called the whole thing simple-minded, just a plain fight between Good and Evil, with all the good just good, and the bad just bad.

...

[Besides], in this 'mythology' all the 'angelic' powers concerned with this world were capable of many degrees of error and failing, between the absolute Satanic rebellion and evil of Morgoth and his satellite Sauron, and the fainéance of some of the other higher powers or 'gods'. The 'wizards' were not exempt. Indeed, being incarnate, they were more likely to stray, or err. Gandalf alone fully passes the tests, on a moral plane anyway (he makes mistakes of judgement). Since in the view of this tale and mythology, Power, when it dominates or seeks to dominate other wills and minds (except by the assent of their reason) is evil, these 'wizards' were incarnated in the life-forms of Middle-earth, and so suffered the pains both of mind and body.

And as to the 'why', he pens this to C.S. Lewis (also documented in the same book as letter 113) where he expresses his own personal opinion about what it means to suffer:

It is one of the mysteries of pain that it is, for the sufferer, an opportunity for good, a path of ascent however hard. But it remains an ‘evil’, and it must dismay any conscience to have caused it carelessly, or in excess, let alone wilfully.