It's like American football, it's a competition sport and fun when you are participating in it. You usually start at a young age and don't think about the repercussions until you get much older.
Some do it for money, others as a hobby. Some like to prove themselves, others are Renaissance men (multi talented)- See Vitali Klitschko, Great boxer, Phd, and a Politician. Pacman, Politician, business man, Turbo Tax expert.
As a (very) amateur boxer of 15 years there are so many things good about it. -Gets you very fit -Teaches coordination and technical skills (unless you are Wilder) -Teaches courage as well as humility -Make great friends in the gym -Gives you an amazing buzz when you win. It's just a brilliant sport.
I believe many do it out of familial tradition. Others obviously because their poor origin. But i know retired Euro champs campaigning for the abolition of boxing out of their frustration with their careers!
Sometimes people are just violent and really like beating people up, with Boxing they get paid for it and don't go to jail.