The sign of a happy, stable-minded, submissive dog is a dog that is not overly excited and that holds his tail low and is calm. It is not normal, nor is it healthy, for a dog to be so extremely excited that his tail is wagging a mile a minute and he cannot stand still. A dog that excited is anxiety ridden with excess energy. Your dog is not sad or depressed.
Dogs are not like humans. They think differently because they are different. A wagging tail is not necessarily a sign of happiness. Dogs that are upset and stressed will also wag their tails. The dogs you see jumping around like crazy, running and unable to stand still are dogs that are not stable. Do not mistake this behavior as being sad or depressed and feel bad for him, or you will confuse your dog by giving off weak energy.