Ah, what a life Jake has – no job, no mortgage and no worries. We take care of all that, and he can relax.
When a dog shows you his tummy that tells you he accepts your authority. Also in Jake’s case it means he feels safe enough in his environment to sleep on his back. This goes back to a dog’s wolf ancestors. The tummy was one of a wolf’s vulnerable spots. To show their acceptance to the alpha wolf’s authority, other wolves would show submission by getting on their backs. If one or more wolves in a pack were not submissive, fights would ensue.
Windy has a cute tummy and likes for us to rub it. Hope, on the other hand, never gets on her back, but yet she does not try to be an alpha. Letting us pet her on the head seems to work for her.