Over the years when I am called into an object guarding case, I tend to see the same patterns. Object guarding is usually caused from hard corrections when the dog is young and possesses an object. The owners yell, scream, scruff grab or even hit the puppy for taking an object that he should not have. They then grab the object away. The puppy quickly learns that his owners want what he has taken and that they (the owners) will become aggressive toward him for possessing these treasured objects.
The first problem is that the dog has been corrected at the wrong time. The best time to correct your pup is when he is thinking about taking the object. The next best time is when he is in the act of taking the object. If you correct the puppy even 2 seconds after the fact and he actually has the object, it is too late. When your timing is off the pup will not understand why you are angry and that the act of taking the object is what you are upset about. All he will see is that he has something you want so badly that you will “attack” him to get it back. Most times when you get aggressive to the puppy he does not know how to get out of the situation. The dog freezes and gets defensive and may show aggression back. As soon as the dog shows aggression and growls or snaps, the owner usually backs away and now the dog has been reinforced and sees that aggression works.
So the best way to avoid this issue is to never correct a dog for having the object after the fact. If you are going to have an effective correction, it must happen AS the dog is THINKING about the behavior. When you see the dog “thinking” of stealing an object, use a “marker” word (like “Phooey” or “Enough”) followed by a punisher (like a water spray from a water bottle on the top of the head) that does not harm the dog. If you have missed your perfect opportunity to correct the dog while he is thinking about taking an object, then trade up and exchange the object using better treats and toys. You may need to initially teach the “trade game” with objects the dog does not care much about, and then work to more treasured objects. So, first practice by setting the pup up. Give him an object i.e. a ball, and say “out” and then show him the treat. As soon as he drops the ball, give him the treat and then take the ball away. Now give him the ball back and repeat this over and over. Now try this with other objects. Soon the pup will realize that when giving an object back to you, good things happen. The act of giving the object back is a good thing. Some people may think that this technique is rewarding the stealing. In fact, on the contrary, it is just rewarding the dog for releasing the object.
If your dog becomes aggressive and goes to attack, then it is best to call an aggression specialist to help you fix this problem. Object guarding can be very dangerous to children and toddlers.
Remember, it is always much easier to prevent these behaviors than it is to fix them after the fact.