I believe the aggression is bred into the breed by irresponsible breeders and owners. I believe that bad situations, bad experiences, bad training, etc., take the bred-in aggressiveness one step further. I believe that while a Pit Bull may be especially aggressive due to his/her breeding, in the right environment, with the correct training, and with a good, caring, understanding, responsible owner, even a naturally aggressive Pit Bull can become a great dog. I also believe that even a Pit Bull with the best breeding (no aggressive lines, no fighting lines, etc.) can be turned into a monster by the wrong owner, or even with a good owner if the dog experiences the wrong things. This is true for ANY breed, including mixed breeds.
An example of such a situation is my dogs. When my dogs (Levi, the Australian Cattle Dog, and Myrl the Australian Shepherd) were pups, they knew few men. My father was busy working so they did not see much of him. The men I introduced them to were good, and they liked them. I carefully and thoroughly socialized them. We had lived at that house for over 4 months and never heard a peep out of our next door neighbor, when one day he got drunk and screamed at his mother, throwing things at her, breaking stuff, etc. The pups heard this and ran to the fence, barking, crying, whining. I tried to calm them down but the screaming, foul words, the sound of things shattering, and a woman sobbing upset the pups deeply. Because of that evening, they hate him. And when he gets drunk and goes into his backyard and kicks things around, they became enraged. They fear him and hate him all at the same time. And he, in response to their barks, becomes violent towards them. We have of course kept them away. If he goes outside the dogs come inside. But they have grown wary of men, and it takes A LOT of effort on my part to get them to accept men now. Levi, in fact, will accept few men. He tries to protect me from them. As soon as he sees a man he begins to snarl. My efforts to show them that the next door neighbor is but one man have not succeeded, even with them befriending men since then. They are still wary of men.