He couldn't be happier. He'd won, hadn't he? All right, he'd had to spend a year as a gnarled, frail, helpless old man in a wheelchair, worrying himself sick about Jack… Martha… the Joneses, and being tormented by the man he'd wanted only to save. But he'd defeated the Master and saved the world and reversed time so virtually no-one would even remember that hellish year.
And so what if in the space of a few hours he'd lost the Master, then Jack and finally Martha?
He'd won. And he was used to being alone, wasn't he? Couldn't be happier.