Working dogs especially require work to feel satisfied. I grew up around red heelers (bred to herd cattle by nipping at their legs, which ours never did) and that instinct easily translates to herding kids or people. They are very bite happy on legs and take serious training and socializing before they are safe to introduce people to. One dog in particular bit most of the people who met him in general. My wife is the only girlfriend of mine he immediately loved when I introduced her. The one before that he was leery of, and the one before that he outright bit. Both ended up cheating on me, while my wife and I have been married 11 years...so honestly he seemed like a good judge of character.
