I'm no expert, but it's my experience that some dog breeds are naturally meaner than other breeds. A responsible and knowledgeable pet owner probably can train and socialize nearly any dog so it is unlikely to attack people or animals. Unlikely does not mean never. Years ago, Austin residents were embroiled in a public argument over a proposed vicious dog ordinance. A young man who opposed a breed-specific ordinance was an eloquent defender of American Staffordshire Terriers, a breed that looks indistinguishable from pit bull terriers.
Breeders claim there are significant differences, but the dogs all come from the same common stock.
He made the argument that it was unfair to pick on certain breeds because responsible pet owners can raise gentle and loving pets regardless of the dog's heritage. His dogs were on the floor next to him. The young man's girlfriend arrived at the house, rang the door bell and started beating on the door. The young man awoke with a start. His well-trained and socialized bull terriers attacked him.