The plain & simple fact is that every breed of dog has been altered by man, at one time there were only wild dogs then they were gradually domesticated, back in the bronze age there is evidence that they had them as pets, then the Egyptians had a lot of domesticated dogs, a type of greyhound, & they are often depicted in their architecture, my point being is that every dog has what is called a "racial memory" & they can revert to their wild ancestry in a split-second at any time, & this doesn't just apply to large dogs, for example Jack Russells have attacked people, a friend of mine who loves dogs went to stroke one on a lead while she was out shopping & it bit her severely on her hand, she had to go to hospital where they treated several deep puncture wounds & she had to have an anti-tetanus injection in her bottom, not a pleasant experience for an elderly woman.