Cause they all think they're right and think everyone else is wrong, and it's so important to them that they can't just sit with people thinking otherwise.
It's really difficult to sit there knowing you're right and everyone else is wrong, even though they think otherwise.
Your automatic reaction is to try and correct them, because you know 100% for certain that they are WRONG.
Now make every religious person in the world think like that and that's where the fighting begins.
Yes, whenever people hold mutually incompatible views of the universe violence is just around the corner.
I remember well the fights that broke out between supporters of phlogiston theory and those who supported chemical theory. The fighting dragged on for years and there were many atrocities on both sides.
I shed a single, manly tear every time I see an economic discussion panel on CNN because within minutes there will be rioting in the streets and shootouts between the Chicago School supply-siders and Keynesian interventionalists.
The worst though, is the endless struggle between the people who see a vase and the people who see two women talking to each other.
If only people had a little more flexibility in their outlook on the world, I'm sure there would be less pointless fighting!