Originally posted by gallaj0
Dogs, and I mean all dogs, are by their nature territorial, protective, and capable of attacking anything they feel threatened by.
People can not tell by looking at a dog wether or not they have been trained to be docile, trained to attack, or not trained at all and left to their own nature. Some breeds of dogs exhibit more or less of these "dog" traits, and will naturally be more resistant to train than others, and more likely to break training for some reason. Pit Bulls, among others, are one of the dogs bred for agressive behaviour, powerful jaw muscles, and territoriality.