Feast your eyes and ears, dear reader, upon the 'tribute trailer' for the Doctor Who 50th Anniversary. It's not the trailer for the episode itself. That is still to come. But interestingly, it does contain a dollop of set-up for the episode.
'I’ve been running all my lives. Through time and space. Every second of every minute of every day for over 900 years. I fought for peace in a universe at war. Now the time has come to face the choices I’ve made in the name of the Doctor. Our future depends on one single moment of one impossible day. The day I’ve been running from all my life. The Day of the Doctor.'
All very exciting, but it does make you wonder what could have caused the Doctor to run for ALL of his lives. We're talking something pre-Hartnell here, evidently. We know that the Doctor stole a TARDIS and did a runner from Gallifrey, taking his granddaughter Susan with him. But we don't yet know why.
However, we do have a crucial clue - and that clue may unravel a lot more of the mystery than we anticipated.
Cast your minds back to the Sound of Drums...
Children of Gallifrey were taken from their families at the age of eight to enter the Academy. Some say that's where it all began, when he was a child. That's when the Master saw eternity. As a novice, he was taken for initiation. He stood in front of the Untempered Schism. It's a gap in the fabric of reality through which could be seen the whole of the vortex. We stand there, eight years old, staring at the raw power of time and space, just a child. Some would be inspired. Some would run away. And some would go mad.
Now, remember what Martha asked the Doctor immediately after this sequence: 'What about you?'
His answer: 'Oh, the ones that ran away! I never stopped.'
I think Moffat may have seized on this throwaway comment from a former showrunner. The Doctor saw something when he looked into the Untempered Schism, that gap in reality that - as we have seen - can open on to any time and place. Whatever it was, it seems to have prompted him not only to eventually flee Gallifrey but to 'fight for peace in a universe at war.'
Bear in mind he wouldn't have been the Doctor then, when he looked into the Schism. It's possible he took the title of The Doctor as a reaction to what he saw.
So, imagine this scenario. A young child of Gallifrey is taken before the Untempered Schism. In that howling vortex of eternity, he sees a moment from his own future. In this moment, he is no longer a boy but a battle-hardened man, waging war on a remorseless foe. Then - to his horror - he sees himself destroy his own homeworld of Gallifrey. He watches the Time Lords burning. Perhaps he even sees the deaths of the Time Lords who are standing by his side as he looks into the Schism. It's his doing. This event WILL come to pass.
That's why he runs from his own people, and that's why he takes on the title of 'The Doctor'. It's a conscious choice NOT to be the 'terrible old man' (to use Moffat's phrase) he saw himself become. He makes a promise to himself to be a peacemonger, a healer, a rescuer, a champion of good. (Perhaps the other boy, overwhelmed by maddening, drum-haunted chaos, chose to call himself 'the Master' for a similar reason? 'I am not that flyspeck, that storm-tossed insignificant mote; I am in control, I am the Master!')
But then, after several incarnations, the Time War comes. The Doctor knows that the Moment he foresaw in the Untempered Schism is coming too. (We know, from Last of the Time Lords, that something called 'The Moment' is what destroyed Gallifrey. The Doctor 'still possesses The Moment, and he'll use it to destroy Daleks and Time Lords alike.')
The John Hurt incarnation abandons any right to the title of Doctor. He acts to stop the Time War, 'in the name of peace and sanity', but in doing so becomes the very anathema he swore not to be. Gallifrey is destroyed and the Time Lords burn.
Once again, the Doctor runs - not from the future, now, but from the past. He buries the memory of the John Hurt incarnation. The entire Time War is locked, the course of events sealed and unalterable. He attempts to become the Doctor again, and with the help of Rose Tyler, he succeeds.
But then, much later, Clara enters his timeline and accidentally forces him to acknowledge his buried past. The Hurt incarnation is revealed once again. That sets events in motion leading up to the 50th Anniversary episode.
We know from the Comic-Con trailer that the Moment plays a part in this. Rose, looking straggle-haired, declares 'The Moment is coming.' One single Moment of one impossible day. A Moment that destroyed Daleks and Time Lords alike. A Moment that, perhaps, the Doctor saw coming at the age of only eight, and which set him to running ever since.
(Cheers to whoisthedoctor at Gallifrey Base for the core idea)