I actually think there is some of both at play here. It is very true that the people who want more of an agressive dog will usually get a dog such as a Pitt Bull.
I believe that most of the dogs that have gotten loose and attacked innocent children or adults, are usually ones that came from this kind of an owner.
The Pitt Bulls that are raised in a friendly family environment are not nearly as apt to just go out and attack some stranger walking by.
However, the natural tendencies of this breed of dog ( as Rebecca mentioned), do make it much more dangerous when it does attack someone.
One of the most likely dogs to bite indiscriminately is the tiny Chichuahua; but even when they do bite, they don't have the ability to do the kind of damage that a bulldog breed can do.
Also, dogs that are perfectly friendly when the owners are home, may be a very different dog when the owner is gone and the dog is left "on guard" of the home.
