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[color=#000000ost_uid0]Regeneration started because William Hartnell, who played the First Doctor, became ill and had to be replaced. (His irascibility also let to friction with new producers, who asked that he be replaced both because he was unwell and because he was increasingly difficult to work with.)

The producers decided it was important to give the Second Doctor a somewhat different personality, so that the new actor wouldn't just by aping Hartnell's performance. So the precedent was established that the Doctor remained the same person, the same entity, but when he regenerated both his physical and mental aspects shifted and renewed.

So it's not too far afield from Dax, although there's no sense of a separate continuing core like the Dax symbiont; it's more like the two Darrins on Betwitched--same character, different actors, slightly different personalities.

But they didn't all just grow old and die out of the part like Hartnell.

The Second Doctor, Patrick Troughton, wisely left the show after three seasons to avoid typecasting. The Fifth Doctor, Peter Davison, followed Troughton's example, leaving after three seasons.

The Third Doctor, Jon Pertwee, and the Fourth Doctor, Tom Baker, retired from the show after five and seven seasons respectively. Broadly speaking there was a consensus between actor and producers that their Doctor was growing stale and it was time for change.

The Sixth Doctor, Colin Baker, was fired by the BBC (and famously refused to return for a regeneration sequence the following season).

The tenure of the Seventh Doctor, Sylvester McCoy, ended when the series was canceled, but continuity was retained when he regenerated into the Eighth Doctor (Paul McGann, who later appeared in the "Hornblower" series) in the TV movie years later.

Starting next year there will be a new Doctor. And yes, theoretically, like all Time Lords he is permitted to regenerate 12 times, in other words to have 13 lives. If need by they can write around that. The Master, after all, has had two incarnations already [iost_uid0]after[/iost_uid0] his 13th life![/colorost_uid0]