How many of you have brought home a new dog…loved everything about it…
and then one day it growled or snapped at you over a toy, or it’s food, or a piece of trash?!?! 

This is a behavior called Resource Guarding. It is a behavior all animals do to some extent. When canines do this in our home the human’s natural reaction is to be shocked, scared, or go on the defensive. Sometimes people yell that their dog “should respect them more than that”, or they cry because their dog “should love them and trust them”… But in reality, all your dog was doing is giving you information about how they feel about loosing their item. When a dog is fearful in a situation it is natural for them to use aggressive displays to let dogs, people, and other animals know that they are uncomfortable. 
So, after the initial shock how do we handle the situation?
Unless the dog continues to pursue you (In which case you would need to seek professional help immediately)…after the initial growl, snap, or bite… take a deep breath.  I give dogs an empty hand signal and say, “Okay, okay…”, then I walk to get them something that would be considered really good to them. You can give them the treat from an overly safe distance. This distance is different for each dog and each situation. It’s called keeping them under threshold.  If a dog is over threshold it is not learning anything, it is just in defense mode. If they are willing to walk away from their item and get the treat, GREAT! Toss more and move toward the item they initially guarded. Keep this up as long as you don’t see them race toward the item, stop and stare at your or the item, freeze in motion, or lunge toward you. If they are loose and happy and you can pick the item up you have been successful! Do not force it! If they let you have it, treat them and have a party! Then give them the item back!…..WHAT? Yes, as long as the item is not dangerous to them…GIVE IT BACK. See if they really care about it now. Trade them treats for it again. Then, GIVE IT BACK. At this point, your dog should be super happy about all of the treating and the item has turned into a happy thing and they like you playing with them and the item. What you have done here is take away the fear of potentially loosing something they value. Some of the training cues that could help in the future are “leave-it”, “drop-it”, “take-it”, and “back up”.
This is a pretty simple explanation of a potentially serious behavior. If at any time you get uncomfortable STOP. A professional that specializes in the issue of resource guarding can walk you through all of the little steps if more training is necessary. The goal is not to teach the dog who is boss or cause a bite. The goal is to get the dog to relax so that if it is ever in a situation that you cannot manage he will hopefully chose to relax instead of bite. Jean Donaldson has an incredible book called “MINE” that could help you understand what is going on in your dog’s mind and how to handle raising a dog that has a tendency to guard things. 
Can all dogs get better?
The simple answer is NO. Some dogs are wired wrong, just like humans. What we do is work on training and managing the issue for the rest of the dog’s life. Different techniques work differently on each dog. Keep it positive. You can’t make a dog positive by adding stress to the situation. Teach them to relax instead of giving them the need to be more fearful and escalate their aggression.