A dog wags his tail in three very different moods and only one is happy. When it’s unusually high and stiff, the dog is agitated and ready to protect his turf. A tail held low and wagged very quickly shows a scared and submissive dog. A happy dog wags his tail in its natural, mid-level position — and his ears, mouth, and body will look relaxed, too. Puppies don’t wag their tails before they are about three weeks old — and some don’t start until seven weeks old. Vets believe tiny puppies are capable, but they’re too busy sleeping and eating to bother. As they become more alert, tail wagging starts as a kind of sign language: a peace sign to rambunctious littermates or when begging for food. Dogs almost never wag their tails when alone. Source: webmd