Disclaimer: I am just playing in the BBC's sandpit.
So it's this time round, the Belgium paradox. The Doctor rubs his jaw and squints at his new features in the mirror. Not a bad face, as faces go, though back then he did remember thinking he was awfully skinny right now.
If he'd told his younger self - what regeneration was he back then, four? Five? - if he'd told his fifth self that he'd reach ten regenerations, and in not such a very long time, he'd have laughed. Yet here he is, number ten, thanks to a very human girl who had saved the world.
He runs his tongue along his teeth. Quite big teeth; big smile. Last time wasn't very smiley - got a bit depressing, that, on occasion. Good to be rid of those ears, really, and nice to have some hair back. And younger, too. Indeed a lot more like number five, this time around. A fresh start.
The Doctor turns, checks his profile one last time, and nods. All is, if not perfectly right, then pretty good with the world. And there's Christmas dinner waiting, with Rose and Jackie and Mickey the Idiot. Crackers, and turkey, and human normality. It's somewhat reassuring.
Somewhen in the future, there's Belgium, and the Master again. But as he has no idea whether Belgium will be tomorrow or in a century, there's really no point worrying about it.