First let's start from a simple point : Newtonian physics provides no explanation for what we can observe, regardless of what position you consider Earth to have in the expansion of the universe.
We have observations showing redshifts that denote expansion at faster than light speeds. That's a key problem to explain if you're trying to develop a theoretical model for the universe because relativity doesn't immediately suggest these redshifts are possible.
So when General Relativity was developed attempts were made to explain the observed apparent expansion away from us in all directions. The key point about relativity is that physics must follow the same rules everywhere. We've found no contradiction of that idea.
The model developed that could explain what was seen was the FLRW metric, named after it's discoverers. This is model has been refined into the Lambda-CDM model over time.
This model explains how it is possible for spacetime to be expanding and specifically requires that every point in the universe will see the same effect - i.e. that they in the center of an expansion. In a way it says that the oldest place in the universe is the place you're standing in - everything you see is younger.
There is no other model that explains what we observe so well. That makes this the model we use, because the gold standard in physical sciences is that the best model is the one that explains the observations best.
The model also provides a vital tool : different distances represent different timeslices of the universe to observe. The further away an object is, the further into the past of the universe we're looking. This provides yet more tests of the theory and our understanding of the universe and how it evolves.
This model ("there is no center") provides not just a match to observations, but predictions about what we expect to see. We expect to see (and do) objects at different stages of evolution as we peer into the distance (and hence the past).
B (unlikely): Earth is somehow at the center of the universe, who is expanding from this point.
We cannot accept this view simply because, while we have a working model for hypothesis-A, hypothesis-B has no model that explains what we see. In science you don't go with an idea that doesn't explain anything when you've already got one that does explain things.
Is there some positive reason to think we are not at the center of the universe
This comes down to your idea of "positive". Most scientists would regard choosing a theory that works over the absence of a theory that works a positive choice. That makes choosing the "no center" model a positive choice.