The Doctor: "An old friend of mine. ...Well, enemy." The Doctor stares, lost in thought. You can be both. It's about intimacy.
Or it's about the sort of pseudo-profound writing that should only appear in college literary magazines. Then, like, 20 pages later:
...and inside the sphere the Dalek explodes, and then there's nothing. And the Doctor stares, because that's him too. And because there's a way in which he's more alone now than before, and he didn't even know it until it was too late. It's about intimacy.
OH MY GOD HE IS REFERING TO HIS OWN CRAPPY CLUNKY CHEESEY POMPOUS LINES! I CAN'T TAKE IT! Too, too dreadful.