I never realized I had this problem until today. I have a 4 year old Pom, Diddy, who's extremely friendly and would never bite anyone... or so I thought. Well today him and our 8 month old gsd pup, Haygl, were having their marrow bones. I gave Diddy the smaller one, but somehow they swapped bones, so I thought I'd go swap it back. Haygl gave me his bone after I said drop it. But when I approached Diddy, he started growling at me. I only just started giving them marrow bones this summer, so I didn't realize I would have this problem. He used to let me take away his rawhide chews with no problem at all.
I was a little annoyed that he would growl, so I hooped a leash around him and pulled him away from the bone. Boy was he ticked. He started growling and even snapped at my pants. Then after he calmed down a little and sit stayed for a good minute or so, I offered him the bone again. This time I held it in my hand while he chewed - no growl, and I could pull the bone back any time I want. But as soon as I gave the bone completely to him, he would growl at me if I come too close. In the end I just confiscated the bone and gave it to Haygl after he had gone back in.
I suspect that part of the reason is that he's learned to growl and snap at Haygl to get his way. If Haygl came too close to his food or toy, he would growl and give him a quick "pinch" on the cheeks. I think that kind of behavior is acceptable, but when he starts using it on humans, then it's a problem.
What should I do about him? I'm pretty inexperienced when it comes to dealing with aggression. Btw, he lets me hold in belly up in my arms and rolls over for belly rubs and such, so I don't think dominance is the issue here.