An aging Doctor #11 (Matt Smith) explains to Clara how he sees himself as his last regeneration. This is why he remains on Trenzalore to defend the town of Christmas from various alien threats, including the Daleks and Cybermen. From Doctor Who S07E15 "The Time of the Doctor"... I do not claim to own this footage, all copyright to the BBC!! I'm only posting this video here because I couldn't find a clip of this exchange ANYWHERE!