Why, oh why?

After slogging through all the posts on the "Violence Problem" thread, I got to wondering what it is about human psychology that demands that people expect everyone else to believe as they do, to the point of killing each other over matters of faith.

It seems to me that if I were a fundamentalist Christian (or Muslim), I would be happy that I found THE TRUE FAITH, and that if I did everything THE TRUE FAITH demanded of me, I could be assured of eternal bliss in paradise. Seeing as how TRUE FAITH's have about billion rules for adherents to follow, I'd be pretty busy worrying about not accidentally eating a piece of pork or spilling my seed on the ground (if I was a guy). If I was a compassionate person, I'd feel sorry for all those people who weren't members of THE TRUE FAITH and were going to hell. I might even become a missionary to try to spread the word to the poor fools otherwise destined for the flames.

But when all was said and done, I'd probably just be concerned with my own little life. Who cares if the people in the next valley are Buddhists or something? They'll be getting theirs eventually. Why is it SO important that they believe as I do? I really don't get this.