Gearheads of War

After reading Papi's awsome diary on XBL, I had a long thought on why people are such jerks on XBL.  Why spend ur time frustrating people because "you were taking their kills"?  Why bother de-leveling so you can kill some new people.  How is this fun?  I enjoy playing the game just the way it was supposed to be played.  Yet, I still cannot grasp how running around acting like a immature little moron brings excitment to ones life.  So, if anybody has an idea of why people do this, please tell me, maybe it will make me that much less frustrated while I am getting team-killed next time I am playing.