Davison has been quoted as saying that Patrick Troughton, the Second Doctor Who, recommended he stay in the role only three seasons, so as to avoid typecasting and becoming stale in the part. And he took his advice. Interestingly, David Tennant - who has called Davison his favorite Doctor - also chose to exit after 3 seasons, though he agreed to do the specials which count as something of a half-season.