Why do some people find it so hard to accept a belief other than theirs?

I’ve been meaning to ask this for some time, and another thread triggered my memory to ask it…

I have seen people get so heated when someone doesn’t conform to his or her beliefs. Why is that? Why do most religions believe it is part of their “duty as a Christian” to spread the gospel? Why does it make a person stupid or crazy if they have a different view of God? Why does it matter if they don’t believe in God? Why can’t we just accept the things people believe? Other peoples beliefs have absolutely no impact on my life, and that goes for every other person in this world! I’m all for discussing things because I think it is fascinating to hear what people think, but I don’t understand why people get so frustrated when people have some “out there” beliefs!

Why can’t we all just get along?!?!