Humans have trained and bred pit bulls to fight and now that instinct is engraved in the dog. Just like greyhounds even if they have never ever been trained still are really damn fast. They were bred to do that and are very good at it. It just takes one wrong move to piss them off. That's why only people who understand how the animal acts and how it was bred should have them.
What?... Do some research and give it some thought? Really? Who does that?