my dog a staffordshire bull terrier keeps on chasing his tail.i would like to know whuy he keeps chasing his tail or is this a natural thing .

It is a known trait in Staffordshire bull Terriers, it is a form of steryotypic behavior, such as seen in zoo animals. It could be being used to blow off steam, is he getting enough mental as well as physical stimulation. It would be a problem if he gets aggresive if disturbed from it, or if he bites at it, otherwise just try distracting him