This seems to be a loaded question at first glance, especially from a evolutional standpoint. We fight to protect our own genetic material and our mate or children. We are thus guardians, protectors to be upheld.
Not everyone who fights is Batman. Some do share his principles, though, of vigilanteism. Those who learn to fight to keep their families safe, and on a greater scale, their country, can be said to hold the oldest motivation for war. This way of life relies completely on the discretion of the person, and the line can be blurry. Would you face down (and therefore be prepared to follow through and kill him) a man who shot your wife? What about a man who only pulled a gun out? Or a knife? Those who live by these rules can be some of the greatest heroes, but they can also be a danger to everyone around them. Vigilanteism is formally discouraged by society, but we still buy comic books....
There are those who will risk it all for the worth of the tradition. In this instance, there's a great amount of variation of opinion between cultures. Some revere these men of honour, and some think them simply inefficient at actual combat. The latter point is upheld by the rise of technology. To use an example of a warfare equalizer: A knight can train for 15 years and be killed by a man who had an hour with a crossbow. Traditionalists are found in most cultures, even those with a proliferation of firearms.
Some people do fight explicitly to fight. To feel bone fold beneath them, to taste sweat and see blood, to hear screams and cries and gasps of mercy. Notice, however, than these are desired sensations that will pass. If you find yourself thinking about this all the time, please seek help. Many will feel this way at least once: your first fight against a good opponent and it goes your way, a battle to uphold a cause that you believe in more than anything. This rage can also be felt in social situations, although it's considerably less encouraged to bite off ears at cocktail parties than it is in a ring. I personally (and feel free to disagree with me on this) don't have a problem with these people. If they keep it to whoever they're fighting and they listen to reason or show restraint when the foe is injured, then I won't argue with it.
I'm sure there are other intentions, and feel free to comment about them. Thank you for your time.
And for everyone interested in alternate workout plans: