The thing about only having one heart is that you don't have an extra for when it breaks.
Here's something that's true: sometimes, when Rose is sleeping and the skies are clear, I go outside and sit on the front steps and name the stars until I lose my voice. Rose thinks it's silly; she'll come out in the morning with a steaming cuppa and make a joke about how old men never seem to sleep.
I laugh, of course. But I think sometimes Rose forgets how old I really am.
It's not that I'm unhappy, mind. Domestic life with the Bad Wolf was something I never even allowed myself to dream possible, back when I was still The Doctor and not John Smith. (Another truth: I'm positively sure that there's no one, not in all the universes that exist and all the universes that have ever existed, like Rose Tyler. She is exceptional. I wouldn't have given up the stars for anything less.) And being Mr. Smith has other perks. I own a house. I have a wife. Sometimes I wear my bedroom slippers out into the garden (I have bedroom slippers and a garden). But I was born a wanderer, I think, or else the untempered schism made me one. (Two types of birth, both equally valid.) Aeroplanes are fun, but they have their limits. I miss my old blue box.
(Sometimes, when I'm out naming stars in the middle of the night, I imagine I hear that familiar whoosh-whoosh and my breath catches in my throat.)
I wonder about him. The Doctor. What he's doing, where he's been. Who's been hanging around in my TARDIS. Does he see much of Donna, these days? (Or Wilf- love him. We had some great times, didn't we?) Has he popped in on Martha Jones? Tracked Captain Jack through the galaxies? Or has he got someone new, someone young to balance out the fact that he's so impossibly old? (I bet he has, the bastard. I never could handle watching them fade.)
Does he still look like me? Think like me? Or has he changed- and in the changing become someone else, someone even more tired and even more lonely and even more lost? I hope not. I like to be optimistic about these kinds of things.
On darker nights, I wonder about other things. (I think about a man called Harold Saxon and his stubbornness, but I never think about that for long. If I think about that for too long- we were the last, the last two, why did he leave me?- I imagine I'll go mad. Well...perhaps that ship has sailed.) (And then I think about Gallifrey and all that screaming and you know what? I'm sure that ship has sailed. I'm mad as a hatter, and who could blame me?)
And on the very darkest nights, when there's not a star in the sky and the moon is just a shining sliver, I close my eyes and remember that once, a millennium ago, I was a child.
The thing about only having one heart is that I know I'm going to die. Not any time soon probably, or at least not by human standards. I've probably got another fifty or sixty years left in this old body of mine. Maybe longer- no one's ever done what Donna did before, and I don't really know what the expiration date is on a Time Lord/human splice. But it doesn't matter, really, because either way my time is finite. And that's different, isn't it? No new face; no new friends. This time when I die, it'll be for real. And I don't think that's a bad thing, even though it's scary and seems a little unfair, all things considered. I think it'll be okay. Because I'm happy here, with Rose and Jackie and Pete and little Tony. My autumn years will be quite peaceful, I suspect. And I'm so very, very tired.