Doctor Who: Why Each Doctor Was Forced To Regenerate

More so than any other Doctor, the War Doctor was probably glad when it was time for him to regenerate.

As the incarnation of the Doctor who fought in the Time War, he was a soldier, a fighter, a warrior, which is a life that pretty much nobody wants to live. His regeneration meant that the Time War was over, and by changing his face, he was able to discard a lot of that wartime baggage he'd been carrying for many years, like taking off your uniform after a long, hard day at work.

The War Doctor regenerates at the end of The Day Of The Doctor, after hiding Gallifrey in a pocket universe with the help of many other Doctors. Much like the First Doctor, the War Doctor has simply grown weary, with his arduous life having taken its toll on his old, rusty body, requiring him to upgrade to a new one.

So that's exactly what he does. The War Doctor regenerates inside the TARDIS, giving way to the very first Doctor of the show's modern era.