Dog Showing His Teeth

Question
I there.
I own a year and half year old Chocolate Lab who has been getting into a bad habit of getting into the garbage, although that isn't my concern.
In the past, when he got into the garbage, I would take him by the collar, lead him over to the garbage and smack him on the nose.
Well, the last few times when I would go to get him by the collar he has begun showing his teeth at me. I of course didn't tolerate it... but at least 3 times now he has continued to show his teeth... even though I'm not backing down.
And finally today, my girlfriend caught him in the garbage, yelled at him NO... sent him to go and lie down, however he showed his teeth and actually growled at her.
I feel like disiplining him is only making him worse. What should we do?

Answer
Discipline is never the answer, with dogs or children.  The answer is fair, consistent, loving leadership, something you haven't been demonstrating.  "Showing teeth" can mean fear grimace (and usually does.)  Dragging a dog to what he's "done bad" and hitting him (and scaring the hell out of him in the bargain) teaches the dog that you are an unstable, unfair and inadequate leader.  You have now created a fear aggression problem.  You need to find a credentialed dog behavior expert to evaluate your dog's temperament and teach you about dog psychology.  Once a dog begins to demonstrate active aggression toward a human, it is unethical for me to attempt to rehabilitate the situation in this venue.  Call the veterinary college closest to you and ask for a Ph.D. or DVM behavior expert who can come to your home and educate both you and your girlfriend.  In the mean time, read Paul Owens or Patricia McConnell, both of whom explain (somewhat) dog psychology and the correct manner in which to live with a dog companion.  Also, go to Karen Pryor's web site and learn about positive reinforcement training.  MOVE THE GARBAGE TO A PLACE THE DOG CANNOT GET TO IT or BUY A GARBAGE CONTAINER with a STRONG LID.  Do NOT strike this dog again.  A growl is a bite waiting to happen.  You're setting this dog up for a serious failure for which it will pay WITH ITS LIFE.