It is widely agreed (among my friends and I at least) that Halo 2 is the most popular game on Xbox Live. My roommates newest interest is recording all our games. So there is an added pressure in knowing that not only are you going to be laughed at once, you're going to be laughed at several times.
There is a very competitive nature to mankind, and it is most certainly reflected through the endless trash talking. Everyone reacts differently when they experience success in multiplayer games.
My friends are no exception to the competition. My experience with Halo 2 is full of frustration because of this alone. I just finished playing a handful of very long snipers matches. It is amazing how much improvement you can make in the game in a short period of time. My roommate is reining champion of our house, but with only a couple months of practice I'm getting headshots on him right and left. Why is shooting someone in the head with a perfectly placed projectile so pleasurable? And why is it so fun to then point and shout, "DOME BLAST!"?