I find that depending on the video game, I can be very laid back or very intense. With WoW and Diablo for example, I am pretty laid back. I can get annoyed at things, but nothing really rage worthy.
Call of Duty games are another story. I dont know why, but I find that I can get pretty damn mad over those games. Getting killed, spawining and getting immediately killed again is a giant pain in the ass. No me gusta.
I think that the real reason for the rage is that I get much more competetive playing those games. I am going against other humans, and I want to win. I want to do well.
I will be the first to admit that I get way too mad sometimes while playing. At the same time, you shouldnt say "It is just a video game". If you are competetive, it doesnt matter. You wont be happy go lucky after losing a football game, or watching you favorite sports team lose. You shouldnt be happy getting your ass handed to you even if it is in a video game.