I was in a fist fight in either junior high or early high school. There was a kid who was actually a friend, but for some reason, he decided he "had" to fight me. He was smaller and lighter and not as strong, so I kept telling him I wouldn't fight him, and I kept asking him what I had done to make him so angry and intent on fighting me.
I never found out the motivation. I finally agreed to fight him at his choice of location, which was in a place where there was no one else around to watch. I showed up and he said, "OK, let's fight!" I told him I wasn't going to fight him, but he threw three or four punches before I decided I really didn't have a choice. I hit him back but held off on hitting too hard. I mostly just punched him in the arm and blocked his punches at me with my forearms. Eventually, he got tired, and I said, "OK, can we call this fight over?" He dropped his arms and walked away without saying a word.
I never did find out what that "fight" was all about.
The only other "fights" I had was the occasional student who decided to strike out at me rather than listen to me. One student was a pretty big kid who had been suspended a lot and was still in his third year as a freshman. He said he was going to leave class and go to the rest room. I told him to wait because there were only two minutes before the bell. He stood up and walked towards the door. I stood in the doorway and blocked it. I told him, "Sit down, Bill." He came right up to me and tried to chest bump me out of the doorway. I put my index finger into the soft spot right beneath the sternum and buried past the first knuckle (it's a very tender pressure point; try it on yourself if you didn't know this). He backed off with me pushing him back with one finger, and some of his buddies in the class started egging him on, "Hit him, Bill! Hit him!" Bill and I faced off eye to eye for a moment with my finger still buried below his sternum. He then broke it off, backed up away from me, and said, "I'm not afraid of you. I'm not afraid of Mr. F______ (the assistant principal and disciplinarian of the school), either." I told him, "I know that Bill. I know that. That's the worst part. You're not smart enough to be afraid of either one of us." He was removed from school before the week was out.
Fist fights are, for the most part, ridiculous and unnecessary. Only fools start them, and only those who aren't smart enough to figure out how to respond in some other effective way get sucked into them, myself included.