Unfortunately, growing up in bad neighborhoods and living pretty much below the poverty level may be the reasons why I have been in more fights than I can possibly count. The reason WHY I fought in those situations was pretty simple....to establish my place in the hierarchy of the streets. Plain and Simple. I fought to protect myself from the neighborhood bullies and (in some cases) to remind others where their place is compared to mine. It is my personal assumption that this is the number one reason kids, preteens, teens and young adults fight in the schoolyard and backyard.
I used to enjoy point competition as a teen and young adult in my early 20s because I was good at it, it was fun and I won way more times than I have lost. For me, personally, it was not so much the thrill of victory or even the thrill of combat. To me it was the feeling, I would even say elation, of accomplishment. Possibly somewhere in the back of my mind I may have also enjoyed the narcissistic belief that I proved that I was better than someone else; meaning the people I defeated in point competition.
Now, as an adult in my mid 40s this may shock alot of people to say that I LOVE Martial Arts but I ABSOLUTELY HATE violence!!!! I LOATHE it! Especially senseless unnecessary violence that could have been resolved by more civilized means. It will take a whole lot to get me fired up enough to engage in mortal violence. But if I am pushed that far or if someone is threatening my wife or kids WATCH OUT!! When it comes to certain things that I must protect I can very, very easily turn that switch on and go into that dark place to defend mine. And I'm NOT fighting to hurt you. I embarrassingly admit that I am actually trying to end your existence and take my chances in court.
OP asked the question of WHY some people like fighting. I cannot answer for every human being on earth but I can offer personal observations and pearls of wisdom I was able to pick up in my lifetime. In the uncontrolled arena, some do it for survival while others do it for sociopathic thrills. Some do it to defend themselves and their loved ones against street predators while others do it because they love being street predators. Some do it to have enough while others do it to have it all.
In the controlled arena, some do it for fun while others do it because they feel they have something to prove. Some do it to test themselves and have goals for their training while others just enjoy fighting....just because. Some people enjoy hurting others. Some may even do it because they would love nothing more than to be able to hurt, maim and mortally wound other people without facing legal consequences and some forms of combative competition allow them to do just that.
TO ME, all of these reasons really point to one reason and one reason alone......that human beings are still a violent, primitive, savage and unevolved race of beings that still have such a long way to go. THAT'S WHY WE LIKE FIGHTING!!
My apologies if that answer seemed too personal, judgemental or forthright but that is my personal observation.
Btw, I stopped competing even during my 20s because I eventually had a personal epiphany about competition and came to view competition as more harmful than beneficial.
Take Care Everyone and PLEASE Have A Great Day,
OSU!