i. immortal
He knows that he will live forever. Existing is a word that defines him. Even when the universe ends, he'll be alive. Maybe not there, but in some other timeline. In some other place.
He hates that. He hates that he can't let go. Living is maybe the saddest thing he's ever done. Living– the definition of a sad, lonely, little man. Everything he's lost. Everyone that died, here and there and somewhere and sometimes even nowhere. That's living. Not joy or happiness or love. Those are just specks in contrast to eternity– and he has an infinity of that.
So he runs. Not away. He runs there, wherever there happens to be in that moment. Maybe, just maybe, he'll trip. And possibly, against all odds, he won't be defined by his existence anymore.
ii. myth
He knows that he's going to die. He's died ten times already, and death is becoming synonymous with Doctor, and not necessarily his death either.
He doesn't want to go. He'll still be alive, of course. Regeneration is a handy way to trick Death. But he won't be himself anymore. For 900 years, he thought regeneration was a blessing. Now, he isn't so sure. What's the blessing in watching some other person walk away with your body? No, he doesn't want to go, but he knows it'll happen, eventually.
So he performs miracles. He saves the world, other worlds, the universe. He saves them over and over again so that one day, even if he is gone and some other person walks in his shoes, a small piece of himself is captured forever.