I tend to see in a lot of movies and animes, probably no earlier than the second or third story arc (or in Battle Angel Alita's case, probably the fifth page or something) the main hero gets asked by a very equal-and-opposite counterpart "Why do you fight?" as a character development question. The answer probably isn't "because my fight against the bad guy is what is pulling the story forward", but rather "I fight for pride and honor" or "I fight so no one else has to" or "I fight because it's fun", or something like that which really spells out a description of the character. So, what about you? Rather than stopping by everyone's Q&A to ask the question so many times, I'll do it here. If you were a fighting hero of some kind, on the level of anime or video game or comic book heroes (and side-character heroes), and you were asked why you fight, what would be your stated reason? I'll start with myself. If I was a hero in one of those media, I think my motivation would be selfishly "I fight for myself, to know that I can". Sure, I would also have friends and my city in mind, but the reason I would fight is to get stronger, so I can know that I am enough to protect those people without having to count on anyone else. Not to leave them to be protected by the other characters, but for myself to be enough, confirming and re-confirming that I am becoming strong enough to do whatever I decide needs to be done.
In that context, my response would be “to provide an example for those who wish to fight as well.” One person can only do so much, but if others follow suit a difference can be made.
I'd probably respond with a shrug and an "I don't know." because I'm neither a highly motivated person or one for giving definitive answers to things. Finding out what I'm really fighting for would probably be a major character arc for me or something.
"...because I choose to." Matrix references aside, I would probably fight for what I believe is right. I know how cliché it sounds but its true. I would fight for what I believe is the best way for the world to move forward. I wouldn't fight for the status quo either, I'd try to fight the system I'm protecting to make real change in the world I was in.
Is it bad that this reminded me of Metal Gear Solid? "What was she fighting for!? What am I fighting for!? .... Snake, what are you fighting for!?" That said, I'm not a fighter, so I would have to be forced - likely to protect someone in the moment.
I'd say that I fight for those who can't fight for themselves. Children, people in minority groups, people who have just been given a bad lot in life. It's really hard to stand up for yourself and what you believe in. It's a lot easier to do when other people are doing it with you.
I'd probably do it, ultimately, to perfect my knowledge about fighting or the techniques, to be honest. Of course, there'd be more reasons, but I think that'd come out on top whether I wanted it or not.