Why do we fight?

What right-minded person would voluntarily put themselves in a position to be hit with punches, kicks, knees, and elbows? Why would anyone choose to risk being cut, beat up or knocked out, with a crowd of onlookers to boot? What does someone gain from dedicating weeks, months, and years to a gruelling training regimen and strict diet, just to have a fight with a complete stranger at the end of it all?

These are, in truth, completely valid and understandable questions. The answers, on the other hand, are perhaps not so obvious.