I like the idea that the Doctor, at the end of his many regenerations, opted for a secluded life on Earth, the planet he always had great affection for. IIRC, the novelisation of Shada has it that some Timelords settle down into some sort of role like this towards the end (in that book, Prof Chronotis, the Doctor's old mentor, is a don at a university). By then, the Doctor has mastered the art of controlling his appearance when regenerating and decides to revisit an 'old favourite.' And he decides to take up that role of Curator that Elizabeth I offered one of his old incarnations.
But like the 'Who knows?/Who Nose!' line, it's left with just enough ambiguity that if you choose to take another interpretation, your reading is just as likely to be correct as mine.