I had a friend once — well, suppose he's still my friend though it's been a while — he started out quite mortal, you see, and then he became immortal. Side-effect of the dangers with time travel, I suppose.

[ The Doctor still has a whole lot of complicated feelings about that, puts some of the blame on himself even though he hadn't directly had any hand in what happened to Jack Harkness. But he'd been traveling with Rose then, and what had happened to her ... with Bad Wolf ...

Anyway. ]

Ah, good. So you're not so lonely, then.

[ Spoken like someone who may have ... personal experience with such matters, of course, not that he'd be so quick to admit it. ]