Doctor Who: Why Each Doctor Was Forced To Regenerate
4. The Ninth Doctor (Christopher Eccleston)
Because it was the first regeneration of the show's modern era, Christopher Eccleston's explosive exit was a visual feast that Doctor Who just didn't have the money (or the technology) to do in the past.
Those beautiful bursts of golden energy are now one of the most iconic elements of the show, and this is where we first saw them (remember, the Eighth/War Doctors' regenerations were broadcast after Nine's).
To get to that point of renewal though, the Ninth Doctor had to stop the Daleks, something that he isn't able to accomplish by himself. Enter Rose 'Bad Wolf' Tyler, who - juiced up on TARDIS energy - manages to literally melt the Daleks into nothingness, while the Doctor watches on in astonishment.
It's a marvellous moment of victory, but on the downside, this power will burn Rose up if she continues to wield it, so the Doctor gives her a big fat snog and takes it all from her, which brings us back to those bursts of golden energy.
The Doctor simply cannot survive this kiss of death, so he regenerates inside the TARDIS, exploding in a flash of bright fiery light. After a few seconds, Eccleston fades into David Tennant, and the fantastic Ninth Doctor is no more.