Domestic dogs aren't working dogs, in the sense that sheepdogs, dogs for the blind, police dogs etc are. Methinks that most examples of "he can understand every word I say" are in fact the result of the dog training the owner, at least as much as the other way round. I've seen the relationship between dogkind and mankind described somewhere as a complicity (you guard my lambs, and I'll feed your pups); later on, companionship hitched a ride, and we arrive at the domestic wolf.

On a related tangent: Many people think that dogs are more intelligent than cats, because it's harder* to train cats than dogs. I believe that cats are more intelligent than dogs for exactly the same reason.

* But not impossible. Our cats know it's feeding time when I whistle for them.