Why is it so hard for Religious people and Atheists to get along?

Yeah, I don't get it either. There's so much circular hate in the world, especially on the internet, between people who would probably get on quite well were it not for their tendency to argue incessantly over beliefs or lack thereof. Life's too short for petty squabbles. Now, more than ever, people should be reaching a hand out to one another instead of biting it.