The essence of the sacrifice of the Doctor is, he would do this for anyone of his companions, ever, under any circumnstances, and regardless of how well he knew them or not. He'd it for Peri whether he knew her just barely or for a while. This is never more confirmed than in the Tenth Doctor's sacrifice to save Wilfred, who's an old man anyway, but the Doctor won't have him die when he can save him - and in Caves, he won't have someone die because of him, because he brought her to that planet. It just doesn't get much better than this, really.