Resource Guarding
A German Shepherd has a bone. A stranger reaches for it. The dog growls. You back off. It happens once, then twice, and years later the dog is four or five years old, at its prime, and it nips you. Why? Because it always got its way.
That's resource guarding — over food, over bones, over anything the dog claims as its own.
The fix: I drop a treat on the other side. The dog drops the bone. I pick it up, then slowly give it back. The dog learns I have the right to give food and take food away at any time.
And listen — if a parent comes in and says, "I can't get my dog's food," that's a serious sign. A dog willing to bite the hand that feeds it needs behavior modification, not just training. I'm concerned from the start.
You are the alpha. The dog can never turn on you.