Possession is Nine-Tenths of the Law of the Wild
People get extraordinarily upset when they go to take a highly valued object from a pup and they are met with a growl. These same people don't blink when a child refuses to give them their doll, yelling "NO, MINE!". But the puppy? Oh my goodness it is dominant, and aggressive, and needs to be punished or disciplined or shown this is not allowed.
We understand today (unlike in the past) that the best way to get a kid to LEARN to share is to TEACH them to share. We are bigger and stronger than the child, but few of us would think it kind or wise to simply grab the doll from her by force. And even fewer of us would consider corporal punishment if she refused to share her doll.
In the dog world, as in a child's world, possession is 10/10 of the law. I want it, I got it, I want to keep it. When we grab things from our puppies, or snatch them away, or do things like pry their mouth open, or force their tongue on to a tooth to get them to open up, we only teach the dog that the rule of the wild holds - the stronger, more aggressive you are the better chance you have of getting and keeping that resource. The puppy that learns force and aggression are the way to lose access to a resource is the puppy that learns to steal, hide, growl and maybe eventually bite to hold onto something he wants.
However the puppy that learns to trade that bone for something better, the puppy that learns to bring coveted items to his owner, the puppy that doesn't worry that his owners approach is a signal that he is going to lose...that's a happy puppy.
Don't raise a resource guard. If your dog already guards resources (beds, food dishes, even people) get a good, qualified, positive trainer to work with you to teach your dog other ways to respond when he has something he is reluctant to give up.
Life is too short to fight with your puppy over stuff, and grown dogs are too fast and too well armed to fight with. Teach your dog that he doesn't live in the wild, that sharing can mean everyone wins, and your home will be happier for it.