If you are referring to debates between people of two different religions, religion tends to be a deeply emotional thing to the believer. There are few things, if any, that they hold closer to their heart and its easy to lose one's equanimity when one feels that the things they hold most preciously and devotedly are under attack. And in many cases, I suspect there is a great fear involved. What if all of the answers they believe in turn out to be wrong? What do they do then? Would life even be worth living?

As far as debates between the religious and atheists? Well the religious side is still operating the same way. And the atheists? Well since atheists aren't unified by a creed and most don't have an emotional attachment to atheism, its hard to make a blanket statement. But many either see religion as being historically destructive or based on flawed thinking, and to them, being confronted with what they might consider to be a brainwashed individual spouting obvious nonsense can be scary too: especially if that person is young or is raising children. To some atheists, the continuation of the beliefs throughout generations is holding society back, and they find that quite frustrating.

And then there is the fact that with some people, all debates end in hostilities.

I find that they often do. That's why there have been so many "holy wars".

I think because as the debate progresses it becomes more heated and before you know it there are insults flying and hostility in the ranks. Its sad how it often comes to that but dammit they hate us Christians!

Because when people engage in a religious debate, they expect it to get heated. Religion is based souly on that which has no measurable form, so it's totally opinionated. When something is opinionated, people can't actually prove anything to have a malleable form, so they often have to assert that their opinions are true even when they can technically prove it.

And so when people assert themselves right above others, that tends to piss the opposition off. Greatly.

Because people don't know how to share their opinion without cursing or insulting people.

Because people can't see past what they want to.

Because nothing makes sense just their opinion.