Well I can't actually say if they fight just for the love of the sport? I mean that has to be part of it with many guys the money is also another part...Here are guys I know don't need boxing to make a good living so they must really love the game...

Wlad Klitschko...The guy has a doctorite for goodneess sake
Calvin Brock...successful banker
Jean Marc Mormeck...Successful Businessman..
Bernard Hopkins...I am sure he started out for just money but heis an all time great now and just boxes for fun IMO

Oscar De La Hoya..Cmon the guy is the richest earning fighter in history

like I said just thoughts