People are intensely prone to various forms of insecurity. When somebody believes something that counters their logic (which is limited to mainstream scientific acceptance in the case of most athiests), they feel that they must protect their own security by cutting away at that belief. Its a simple psychological concept.
On the other hand, I don't mind. Our brains aren't capable of containing the greatness that is the inner-workings of the universe. We're still evolving :-)