I've said it before...it all comes back to that restrictive, "one, true way" BS they're all taught.
When you are told that your way is the only way, and that you have all of the answers, then when someone inevitably comes up with a different answer, it forces you on the defensive. You are forced to either defend your mindset by attacking the contradictory evidence, or actually consider it for what it is. Guess which one is more likely to happen?
Also, people like feeling that they're special, that they are somehow above the masses. Christianity feeds that need, telling them they're special, better than those who are not Christians. So of course they feel the need to shove this perception in the face of others.
My advice? Start hanging out with pagans. For the most part, we care a lot more about your ethics than your religion.