In a study conducted by researchers show that gamer aggression can "mimic that of warfare". The study involved teams of three playing capture the flag mode in Unreal Tournament. The results showed that levels of testosterone spiked after one group defeated the other, especially those with higher scores.

"In a serious out-group competition you can kill all your rivals and you're better for it," said David Geary, an evolutionary psychologist at the University of Missouri. But when competing against others in order establish a social hierarchy, annihilation doesn't make sense. "You can't alienate your in-group partners, because you need them," he said.

Although there were cash prizes involved, I would think that having something to prove to others sometimes does fuel your gamer aggression more than anything else. That's from experience. I play Call of Duty: 4 Modern Warfare online a few nights out of the week, and I realized a lot of the times I get a good score because of my drive to prove that I can f*ck you up. Sometimes when I play with a group of friends just makes it much more exciting and challenging too, because I feel the need to step up the game, and the need to play better than everyone else. All of those things (even though they may sound self-glorifying) really affect your gameplay. At least for me anyway.

Via: Kotaku