MINE!!!!

With Resource Guarding, particularly with food, toys or bones, we must realize that a dog is only doing what comes naturally to Dog.

Protecting something that may seem incredibly important to your dog, is something he may have learned well before you brought him home.  He may even have been 'fighting' for the resource of momma-dog's milk; learning to growl and/or snap to get his way as the litter grew, and by the time you chose your sweet little ball of fur, he thought that was the way to keep what you wanted as Yours.

Or perhaps you have an older dog you adopted and discovered you and your family are suddenly faced with Cujo after you gave him a tasty bone.  The shelter said he had shown no signs of it, so this is 'new' to you.  There is not really any way to look into his past, but what you can do is help him with his future.

It's all about breaking it into smaller chunks, and finding what means more to your dog.
Toy..........................or bone?
Bone...............................or boiled chicken?
Boiled chicken............................or rotten squirrel he found in the forest?  (ok, that last one may be a touch harder to work with, but you get the gist.)

The following link will take you to Dr. Sophia Yin's blog article on the subject of 'food aggression,' and will show you what I find to be the most effective way to help a dog with this kind of issue.  Not all dogs benefit from this approach, but the majority of cases I see can be turned around with something similar to it. You must cater the method to suit the dog and the particular circumstances.  It is an excellent framework for using a more scientific approach (Dog is Animal; what does Animal want?) to help a dog overcome the feeling that everyone is out to steal his things.

We're working with one canine now, who is simply learning that going "crazy mean" in the kennel with a raw bone is just not necessary, nor are we scared and leaving; but rather, he is learning to calmly go in, chew the bone a moment, then come back out calmly and receive pieces of chicken.....

So, enjoy the info:  http://drsophiayin.com/blog/entry/treatment_of_food_possessive_dogs_is_about_finesse_not_force