Dogs like to bite their tails because there are foreign objects, itching or injuries on their tails

When dogs have foreign objects on their tails, itching or injuries, they also want to see what’s going on with their tails question.

Dogs like to bite their tails because their anal glands are blocked

Once a dog's anal glands are blocked, because the tail cannot be easily touched, you have to bite your tail or rub your butt.

Dogs like to bite their tails because of obsessive-compulsive disorder

Except for the above pathologies, they will still chase their tails, maybe because they are bored. If this happens Just give him a toy to play with.
However, dogs that leave their mothers at a relatively young age are more likely to develop obsessive-compulsive behaviors. Such dogs are more introverted and less aggressive toward humans than other dogs.