OK, so Dinah inspired me with her "You're The Psychiatrist...." post. She does do this fairly regularly. She stumbled into an Ultimate Fighting event and came out wondering, "Why do people do this?"
I'll tell you why. I have some experience with fighters, both as a psychiatrist who works with violent people and as someone who has hung around black belts for about twenty years.
It's about competition, it's about adrenaline and excitement, it's about taking risks and not being afraid of the consequences. (I'm tempted to say 'it's a guy thing', but besides being a sexist comment it would also happen to be an untrue statement. At some of the martial arts competitions I've been to I can tell you there are a substantial number of women competing nowadays. And you should see their tattoos!) So it's a sport, although I have to say there's sometimes a fine blurred line between a sport and a crime. If there are rules, if there's a professional organization sponsoring the event, if you have to pay to get in and you get some kind of formal training, then it's a sport.
Then there are crimes. People who fight---without rules and without sports equipment----sometimes do it because they enjoy it. It releases tension, gets rid of pent up emotion, and sometimes it settles problems (whether it's a good way to settle problems is obviously a whole different question). Among prisoners the challenge is to see how "good" you are at it or to establish dominance and defend your turf. It's to enforce gang rules or to punish rulebreakers. Among the younger inmates (also called "hoppers" in prison slang, after hip-hop) the idea is that fighting is protective; by being willing to 'step out' you'll be less vulnerable and it will keep people away from you. Younger inmates also will prove themselves by going up against much bigger prisoners or correctional officers. (The much bigger, more experienced correctional officers can usually see this coming and can 'talk them down' or persuade them that it's really not a good thing to do.)
So that's what my experience has been and what I can say about the motivation of fighters. Street fighters eventually grow up or burn out. They figure out they won't always be the biggest baddest person on the block and that injuries accumulate over time. Then there's the rare person who never figures it out, and they stay locked up. One prisoner I met had been in a coma for several weeks as result of a street fight. I asked him what he had learned from the experience. His response:
"Next time I bring a gun."