Because it's like politics for children...
But seriously, I think there's 3 main factors involved.
1: Basic human psychology. People will associate themselves with a community, whether that's the "PlayStation" community or "Xbox" community. Once this happens, people will try to support the community they are part of.
2: Justification. People want to convince themselves that their console choice was the correct one, so they'll try bashing any competitors to ensure this, even if they're living in delusion. (Most of the time a troll says "I own both, and the other console sucks!" They are simply lying)
3: Peer pressure. Because so many people fight over plastic boxes, people feel pressured to join the fray with everyone else.
And it's not just consoles, either. You can find this exact stuff in other areas. Democrat vs Republican, Intel vs AMD, Nvidia vs AMD, Apple vs Android, and also many sports.
EDIT: It's also wise to not get yourself involved with console wars in any way, even if you're just trying to prove that someone is acting stupid. Refer to my sig.
"Never argue with stupid people. They will drag you down to their level and then beat you with experience."
-Samuel Clemens
