They can do it out of boredom , to entertain themselves, especially if they are puppies.
If the dog sees that his owner is amused by his tail being chased, he may be doing it to please him.
According to a study by the University of Helsinki, tail-chasing dogs are often shy and frightened by noise. They also found a relationship between the act of chasing one's tail and the OCD suffered by people. The research found that dogs that take vitamins with food are less likely to have this behavior. If your dog is compulsively chasing his tail, take him to the vet. Something may go wrong.
