To be honest, I think the cause of most of the flame wars... in general, I mean serious flame wars where people make points and not just yell, where some actual thought has gone into the arguments, is two fold:
1. People love the system they have and feel the need to defend their purchase and / or decision.
2. Most of what the other people say about the systems are lies. When someone says the 360 RROD's a lot, I know it's true, but it's not as horrible as people make it out to be, I like having an educated discussion about that aspect.
However, when someone says "Live is filled with 12 year old high voiced whiners and college frat boys", I have a problem with that statement because it's a generalization and is just not true. That would be like me saying "PS3 is for graphics whores to show off their TV's". It's just slander. And everyone hates slander.