I'm kidding (sort of). Retrievers are of a group that tend to be obsessive about mouthing and chewing. Little pups are merely exploring their world with their mouths. Actual teething doesn't really begin in earnest until he is 4-5 months of age, and lasts until his adult teeth are fully in. But that's not the end of it--not by a long shot.
What they say about Labs also holds for Goldens: "They chew 'til they're two, and they shed 'til they're dead".