Why does a disagreement in perception lead people to seek solace in hate?

I've been on almost every conceivable side of the religion argument at some point throughout my lifetime. Currently I have come to the conclusion that 99% of religions have the same basic premise at heart. That being said, why is it that almost every person who has claimed belief in some higher power gets so angry when they come into contact with some one who doesn't believe in one?

Religions are supposed to be about bringing people together who have similar values, and creating a community around those beliefs. They are supposed to love their fellow man, and help to end other peoples suffering for the good of humankind. Again, these are the beliefs almost EVERY religion holds dear (and I would assume most any sane Atheist as well)... yet when you express to them that your beliefs differ greatly from their own, at least in my own experience, they treat you like you have a disease. If suspension of belief is contagious, then I surely hope I can pass it on (as long as it works for the person who is infected).

My main problem is this... I don't care what someone believes or chooses to do as long as I am allowed the same courtesy, and as long as it doesn't hurt or suppress another individuals will. What is so difficult about that? Why MUST these people with their beliefs be so militant (Atheists can be guilty of it too) and attempt to replace another persons ideals, beliefs, and morals for their own. What makes us want to either through coercion or force, cause some body to have a perception shift whether or not they are prepared for it?

I look forward to your responses.