I think fighting is a part of human nature, everybody sometimes wants to fight. This is on a small scale, but when politics get involved, it starts to get on a much more greater scale. People will start to feel hatred for the "opposing" faction. War comes -> people die -> to make it look like they didn't die for nothing, people will gloryfie them and their actions + the politicions will do it aswell, so they have the country behind them and make them believe in false or real values. For example: the guys now fighting in Iraq, everybody sees that it's rediculous and they aren't doing shit over there, but the americans won't say a bad word about their boys, they will even glorify their acts.
That's how I think it starts, but now a days you love the fighting because it's all around in our media. We see films like 300, the deer hunter,... memories of fallen soldiers, hero's... And we think, if I fight I will become somebody, people will look up to me, I get all the girls, and if the worst thing that can happen happens (you die) you will be remembered as a hero for eternity (which is a pretty good deal if you ask me). So that is why average joe sings in the army and gets his head blown of after 5 seconds of combat.