Seriously, there are two reasons, closely related

1) Religion (and probably sex) are the closest and most personal things a person can think about. Religion can be the core of a person; it encompasses their set of values and beliefs. An attack on a religious thought is seen as an attack on that person, and that in turn is seen as something to be defended - to the death, if need be. You attack a religion, you are attacking the core of a person's self, and that causes controversy.

2) Religion is closely related to power - in many religions, the most religious have the most power. Power corrupts - no question about it. So the powerful look for ways to assert their power, because power that is not used is not power at all. So religious leaders, convinced of the rightness of their beliefs, use their power to force religion on others. (see: Crusades, any number of ethnic and other skirmishes, etc.)

Discussing it one-on-one at work, or at a party, is just a microcosm of the points above. If you questions your friend's religion, you are threatening his core identity.

People have killed, shunned, ostracized, and sacrificed themselves for it. Why?

Because humanity finds it hard to believe there is no god. This is one obsession we cherish.

Cuz it's got a lot of people sensitive to their diety, and the believe that a heaven awaits the righteous and abyss for the unrighteous, sends chills down their spine. So when you make a joke about it or you say they're doing the wrong thing, expect them to go berserk on you even if it takes their life. They still hold the belief that their gods will defend them.

its becos people foolishly thinks they can fight for god