Why do people get so angry about religion?

[pilotedstate]

Why is it that when you just try to have a peaceful conversation about religion, and your beliefs conflict with someone else's, people tend to get extremely angry? People start shouting in anger and even get physically violent just because someone is talking about a belief different than their own. Someone may say that they are a Christian, but when their faith is opposed, they start swinging... I just don't understand this at all. The same thing happens when people are discussing politics... it just seems so ridiculous to me that these things cannot be discussed in a peaceful manner, and I want to know why people get so angry and violent from a normal conversation.