People learn early on that they're going to be judged on the basis of how many of their opinions are correct. That's largely what school teaches us -- if we're right more often than someone else, we are better than them.
People tend to freak out when somebody disagrees with them. They're worrying that they're WRONG, and to forestall the terrible possibility that they might be wrong, they immediately set about trying to demonstrate how wrong the other person is, so that they can feel good about themselves for being right.
It's, as someone already pointed out, all about ego. Or rather, the inability to separate one's ego from one's ideas. So many people find that separation very difficult to achieve (largely because our education system teaches us that that separation doesn't EXIST), and it's truly debilitating. It inhibits intellectual growth and traps people in a terrible spiral of waste and suffering.
Which is great, because then I get to dissect their posts point-by-point and demonstrate what fools they are in an amusing fashion.
...
I mean, which is terrible. Very bad.
I certainly don't think it's restricted to gamers. I've seen this behaviour well-established among software developers, marketing professionals, journalists, housewives, waiters and loggers.