I fight because I enjoy it. I'm a martial artist and to me, nothing is more thrilling (except maybe sex) than fighting. Its a rush, the feeling of adrenaline pumping through your veins while you defend your body from harm and possible death while trying to overcome your opponent physically and mentally with whatever tools you have. You're reminded that you are alive when you are faced with your own death.
I fight to defend myself, yes. I fight to defend the lives of those I love, or those in need. Other than that, I won't fight for any reason beyond the sheer enjoyment. Its fun. Its a game, a sport, and life or death all in one. And while I haven't been in a life or death fight before, and hopefully never will, the simulation of the act is thrilling. And in victory, we gain things like honor, prestige, and respect from our fellow man. We've proven our strength, speed, skill, courage, and worth all in one act. Its been said that you can see a person for who they truely are when they fight. It brings out the best and worst in people, and shows who they are deep down inside. You'll feel a full spectrum of emotions during a fight, and in the end, you're left feeling truely alive no matter what (unless you got killed, heh).
As for war, war is the cause of human ambition, prejuidice, and misunderstanding. We war because we fear one another, we war because we want what each other has, we fight because we are different and will not accept those differences. War is the outcome of humans being human, pretty much.
Before there was war, people fought for survival against one another. We needed what the other man had, be that his possessions, his food, his land, or whatever. As time advanced and people began to form tribes and societies, this turned into war. However, the one on one fight stayed as a institution for men to feel that feeling of primal, life or death survival. We'd fight to feel that rush, and to protect our honor. We'd fight to take the name of another man, to show that we overcame a man you either disagreed with or who was reknown for being a great fighter, to show how great a man (and fighter) you yourself are.
I love fighting because I love that feeling of being alive that it gives me. I don't fight to say that I beat another man, or to protect my honor. I fight to protect my friends and family, and myself. But if that is not there, then I fight just because I love the feeling of it. It's intoxicating. Humans have a love-hate relationship with death. We hate to fall to it, but we love to overcome it. And when we fight, and win, we have overcome death.
Atleast, for the time being.