Snapping

Question
Hello.  Our male, 4-year-old black lab mix has started snapping at people.  People who have been able to pet him now have to back off.  He has been to the vet and there are no physical problems.  He will, sometimes, spin around on me when correct him.

Answer
I'm unsure what you mean by "when correct him"...are you using physical correction (choker collar, hand, etc.)?  No dog is a fair candidate for this sort of "correction" but this one is especially NOT.

Dogs learn about aggression because it works; dog snaps, person backs off, dog is rewarded.  Your dog may be demonstrating fear aggression or even (unlikely, but always possible) dominance.  (Dominance aggression is actually quite rare and occurs in intact males of certain breeds.)  It's impossible for me to determine how this began but it's obvious that it is progressing.  The "spinning" you describe could be the result of anxiety, fear, anger (yes, dogs get angry) or even cognitive dysfunction brought on by high emotional state (which would explain his developing aggression, also.)  Any actively aggressive dog requires the in person evaluation of a certified applied animal behaviorist.  This is NOT a problem that can be addressed in a text box; it is not only unethical to do so, but might precipitate a serious injury to a human.  Contact the veterinary college in your geographical area and ask for referral to a Ph.D. or DVM with an expertise in treating active dog to human aggression.  Until then, don't allow this dog to greet people who come into your home.  Each time he snaps and a person withdraws, he's learning more about how to use aggression to control his environment.  The time is rapidly approaching when he will connect and injure someone.