Sometimes, dogs bite themselves because their skin feels itchy, like when they have dry skin or allergies.
Other times, it could be because of tiny bugs that live on their skin, or even because they have a sore joint.
If your dog keeps biting himself, it's important to take him to the vet to find out what's wrong and get him better.
The vet might give him special medicine or a special collar to help him stop biting and feel better.