For fandom_muses: "I have always depended on the kindness of strangers."
“Monsieur, what are you doing in my fireplace?” No impolite scowls, no tainted disillusionment that was fairly well-bred into the century from which her French vowels came. She was kind, to a strange man crouching in her fireplace with a lot of questions.
Really, though, did the Doctor not always depend on the kindness of strangers? Ask them where he was, what year it was, silly questions like that. In return, he saved them, gave them free lives, protected them from dangerous things like clockwork men, Daleks, and burned toast.
How was he not supposed to want to help her? Figure out why the hole was punched into her world. With a click of the fireplace he slid from his own world into hers, unknowingly imprinting himself on the child in a way that only a heroic stranger could. Armed with a blue wand that could light candles and destroy evil clockwork men, he was her secret friend, something so few child aristocrats could have.
“Monsieur, be careful!” she cried out as the clockwork monster’s knife shot out.
“What do monsters have nightmares about?” All innocence, all smiles. He was her hero, her protector, this kind stranger.
In ten years her time, only ten minutes his, she was young and beautiful, soon to be wed, and looking up at him in a way that begged for him to be impressed with her age, her beauty.
"I think it is proper for one to have an imaginary friend only during one's childhood, you are to be congratulated on your persistence." The cool arrogance slipped from her voice with a practiced air---she was trying to impress him, use what she knew.
In a manner more befitting of a woman her age several centuries later, she pressed her lips against his, solidly, firmly. Somewhat a shock but more of a kindness---it had been so long since someone who genuinely loved him actually kissed him. Perhaps she saw that in his loneliness, knew he was secretly asking for a future French aristocrat to love him.
And she did, love him. Another four years for her, ooooh, around fifteen minutes for him, and she had seen inside of his head, and still desired a dance with him. Her fingers entwined with his, a slow seductive smile spreading across her heart-shaped face. Pity? Perhaps. Presently persuading in the form of a proposition was perhaps a piece of pity---but the Doctor takes it, follows her. Allowed his foolish 900-year-old hearts to love her.
Threw himself from his own time stream to save her. For she was no longer a stranger, now. A piece of him rested in the 1700's of France, and he had to save it. Lose the TARDIS, lose Rose (lose Mickey, too, but, well...)---all for a stranger with a childlike smile.
It would've been worth it. Saving her would always be worth it.