I know a lot of it is from what people did. When I was a kid in the late 80s/early 90s, we lived in a town which had many illegal dog fighters in it. They mostly chose pitbulls but not solely. We ended up saving one puppy from them and he was some sort of mastiff mix.

One of their pitbulls escaped once. I opened my door and it came running from out of nowhere, snatched my cat from beside me and shook her to death in front of me. It was so terrible.

They would do the worst things to these dogs. They would beat them, use food against them and for a long time it was even really popular to feed them gun powder to make them more aggressive. They would condition their dogs in any way they could to make them good fighters and as aggressive as possible.

Pitbulls have a lot of sharp teeth and sturdy, muscular bodies, so they never had a chance with these assholes.

The dog breed I've personally seen get out of line as a pet and bite people the most is the German Shepard. That's just my experience. Either way I believe that dogs are like people. They can have genetic tendencies but their environment will have the largest influence on them most of the time. I've never met an aggressive pitbull outside of those terrible dog fighter's dogs.