Why do dogs chase their tails?

BOBBY CAWTHRON, DVM: Some tail chasing can be normal play or grooming behavior. It can also be a response to an injury. Consistent tail chasing, however, is abnormal and can fall into behavioral concerns of attention-seeking activity or OCD (obsessive compulsive disorder). These abnormal behaviors are likely reinforced by an endorphin release causing a pleasure response in the brain.

source: https://aspengrovevet.com/