Dog Aggression, a learned behavior
When a young puppy snaps and growls, people laugh or smile, and say he's cute or a tough little guy. When in actual fact often times its the first step down a slippery slope. Yes puppies do bite, and its not a mean behavior or anything to worry about. But it still should be stopped. If a puppy lunges and snaps at you when you go near his food bowl or favorite toy, it needs to be addressed. A cute 6lb puppy snapping and bearing his teeth around his food bowl, isn't intimidating, when its a 80lb adult doing the same thing it is.
Once a dog practises using his teeth and intimidation in one avenue, it very likely will explore its success in other areas too. If he snaps at your hand going near to his bowl and you move your hand away, it worked. So later when you want to cut his nails, and he doesn't, snap and you leave his foot alone. He is training you. He's learning what works and what doesn't, he learns who it will work on and who it won't. He is practising. And practise makes perfect.
The good news is, it nearly always can be reversed. The longer they have practised the behavior for, the longer it usually takes to reverse it. There are of course other variables involved.
This is why puppy classes are a good idea, a 3rd party can see your dogs behavior, and not through rose colored glasses. They can give you practical advice. If there is no class near you, then my advice would be, if the behavior your puppy does now is what he does as an adult, ask yourself will you still like it? if yes, thats your decision. If no, fix it now before he's perfected it.