Maybe it is sick, maybe not. It could be depression too. I had a dog that once went depressed, the vet said the dog was healthy. But it was obviously happy to be out, see new people(the vet, other dogs at the vet), ect, which makes it hard to diagnose. You can walk a dog so much, and give it so much attention, treats ect. At some point you just have to run down a list of activities that are more common for that breed, see if you're missing anything important. In our case, we got the dog a cat to play with, because the breed I had carried a sense of nurturing that didn't bode well with the highly independent people that it lived with.