(Something to chew on, as a masticatory exercise for the reader. And apologies in advance - this is a breedist post, based only on old reading about pit bulls and no personal experience with them. Perhaps they're just more susceptible than (some) other breeds, to influence from bad upbringing.)
The problem with pit bulls is that they either don't recognize, or don't care to heed, the normal social "ok, I'm not fighting you" body-language message sent by other dogs - once the other dog gets assigned the "enemy" role, that's where it stays.
I think we've all known people that act the same way.