It's very common for a dog to protect his prize resources. The good news is that "resource guarding" - growling, snapping, biting over food, toys, socks etc. is the easiest kind of aggression problem to solve in dogs, unless you use punishment. Watch this video for some useful advice.
Hint: It's not a contest. Using force can backfire and lead to people getting bitten, especially kids. The trick is to teach your dog that sharing is awesome.