On Reason

Why is it that reason does not seem to always be enough to justify a belief? Why is it that you can have reason contrary to your belief? Is it your brain? Does reason have nothing to do with belief? Is reason an illusion? (I had a class about this once- it was really funny that we reasoned for three hours about whether or not reason was an illusion). And I mean by that- Do we always make decisions based on things other than reason? Or is the disconnect between belief and reason (which we sometimes experience in our own lives) due to something else?