Mike Tyson may be the exception to this rule.
He was made fun of and was afraid to fight at first.
But then, apparently Tyson owned pigeons as a kid. Some guy from his neighborhood took a pigeon and said to him "You want your pigeon back?", and then proceeded to pop the pigeon's head off. Tyson then proceeded to beat the shit out of him (~6 mins into the documentary).
He was also so scared during his first amateur match that he thought about grabbing a train and leaving (minutes before the fight).
Watch "Tyson" on Netflix (streaming). He tells his life story, first person. Surprisingly moving and not a sensationalized account like you'd expect.
"One morning I woke up and found my favorite pigeon, Julius, had died I was devastated and was gonna use his crate as my stickball bat to honor him. I left the crate on my stoop and went in to get something and I returned to see the sanitation man put the crate into the crusher. I rushed him and caught him flush on the temple with a titanic right hand he was out cold, convulsing on the floor like a infantile retard."