There is always the question. A possibility, though not great; he's not the sort of man to do this casually, but he's done it before and will do it again. And it would probably be an abuse of something to go to bed with Rose Tyler, but his people are gone now and if he can erase two entire species from the history of the universe without everything going pear-shaped, he can probably get away with some ethically dubious sex. He's her teacher and her father figure, but then there's a reason humans write songs about that sort of thing.
So he considers it. Carefully, deliberately, though there is a sense of inevitability to it. This regeneration doesn't let people in, doesn't do domestic, but Rose is the exception to all the rules, somehow, and he knows he'll give in eventually. He even flirts a little, though he doesn't expect her to press the issue, pulling him into that awkward half-waltz that leaves Jack leering knowingly and the Doctor's hearts thudding in an uncomfortable syncopation.
A few weeks later and he's nearly decided to shag her, except then he's in Cardiff, wining and dining a Raxacoricofallapatorian of all creatures, when the backs of his hands start to itch, as if he's wandered into some kind of temporal loop. He looks out the window to see himself, only in a new body and a sharp suit, nattering on pleasantly and towing Rose after him. No Jack, but the Doctor's not surprised at that. Jack can look after himself. He's more interested in this other-Rose, no more than a year older, beaming up at the new Doctor and threading her fingers through his, not as a daughter or a student or even a lover, but as a girlfriend, as though the Doctor's some ordinary bloke, a replacement for the hapless Mickey.
And this is why the Doctor hates knowing his own future, because now he knows that he and Rose can never have sex. He won't do this to himself, won't saddle this older (younger) version of himself with a lover he never chose. His doppelganger is gangly and too young and looks a bit like a weasel, but he's the sort of man Rose Tyler could fall in love with, and even the Doctor isn't cruel enough to let that happen. Rose looks at him and thinks of shagging; his current body's not good-looking, exactly, but the cheekbones let him pass for handsome and he looks just old enough to make the sex seem forbidden and thrilling. She fancies him in that pure, uncomplicated, human way, and he'd shag her if only he didn't know she'd look at his next body and think of marriage.
No matter what the Doctor's next regeneration is like, that can't end well for Rose. She'd want a house and a garden and an ordinary life watching matches down the pub, but the Doctor has never been good at that sort of life. Rose knows too much, he realizes with a sinking feeling, knows about humans and how someday they'll dance their way across the stars. She'd want children, and she's clever enough to guess he could give them to her. He's not sure what scares him more: that Rose might ask him to father her children, or that his future self might agree.
He remembers being a dad once, remembers raising his children under a burnt orange sky, and, later, his granddaughter on the adopted planet they both loved so well. But memory cheats; there was never a planet called Gallifrey, never a race of pompous aristocrats who called themselves the lords of Time. There was no girl named Susan to make her teachers wonder at her unearthly knowledge, and those teachers never followed her home and aboard a spaceship whose very existence is impossible. He could go back, find Barbara and Ian, and see if they remember a strange girl who lived with her grandfather in a police box, but knowing makes it real, and he's afraid to find out for sure. He's afraid that none of it ever happened and his friends lived out their entire lives in one boring, linear stretch of time. He's afraid of being alone.
And now his Rose stands before him, glowing with borrowed power and dying in that stupid way that humans have, so he reaches out to kiss her and change himself into someone new. He swore he wouldn't do this, but she'll never remember anyway, and even the prospect of death hasn't stopped this body from wanting. It's the only liberty he'll allow himself here at the end. Let his replacement make his own mistakes. He has no desire to remake his civilization in his own image or start a cozy human life with this girl who will outlive him. He's found the answer to the only question that matters: how to keep Rose safe. For this body, this lifetime, that's enough.
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Author's Note: Title from "The Sonnets to Orpheus" by Rainer Maria Rilke as translated by Stephen Mitchell.