Usually when cats chase their tails, they are simply playing. Cats are not being fooled; they know their tail is theirs. They often groom it when they're finished having fun. The tail is just a handy toy.
However, certain medical conditions sometimes lead to tail chasing. When there's a physical problem causing the behavior, cats often seem frantic or irritated rather than good-humored. For example, fleas cause intense itching. If cats appear to be chasing their tail crazily, they may really be trying to reach an "unscratchable" itch. Cats sometimes chase their tails because of allergies to substances such as food, pollen, grass or mold. Occasionally, cats react to stressful situations by chasing their tail. They perform the activity in an effort to relieve their anxiety.