It's difficult to imagine: the Doctor being in love. The way she sees it, it's like assigning human characteristics to something inanimate like a mountain or sunrise the way the ancient Greeks made Gods out of concepts. Except that when it comes to being animated and feeling: the Doctor epitomizes both; he's not a concept (much as he would like to be). He's no human, certainly, and this can inexplicably make it seem like he has supernatural powers when all he really has is a wondrous and vaguely ridiculous timeship, a motormouth, an intellect larger than the largest planet she can think of and a backbone made of something forged in the heart of a star.
It doesn't make sense, not really, Donna knows that, and she's fully aware this line of thinking is something the Doctor has put in her head. That he's above those foolish things, because, after all, a sunset or a mountain (something magnificent but far away) would never love anyone, but you'd never expect it to. There's only one thing that reminds her she's wrong: the way he shies away whenever she asks him about Rose. The way his face looks, just for the half-second before he deflects, changes the subject, steamrolls her with suggestions or facts or regales her with a story about a revolution on a moon colony a hundred billion light years from home. It's the way his eyes are open wounds in that moment, and she remembers his voice shaking when he casually told her the name of the girl who had left him (this blustering force of nature that could stand there and watch everything die in a whorl of water and fire with no expression) looking like a sad child, this ancient greek god: a walking, breathing personification of concepts too difficult to explain. The Greek god of solitude. The god of impulse. God of restless, formless anger.
It's something she forgets about until she sees it, how it passes over him like a shadow, a cloud covering the sun when the wind whips up a gale. She sees it and remembers this thing that brings him closer to being like her, being the same in at least one aspect: that they could be broken. That all it took to shake the heavens for god or man was something precious lost.
The Doctor, the Greek god of lost things.
This is what Donna remembers when she tells him, in reply to his irritated question, that maybe he should ask her himself. She remembers it when he stills, looking stricken, wounded eyes laid bare and painfully hopeful for that second before he turns and runs, sprints, pays their surroundings no attention and then pays for it. This is what she remembers when she watches him casually shrug off the cataclysmic event of his almost-death, when the somber blonde from the parallel timeline throws her arms around him and holds in a way that Donna would never. She buries her face; he closes his eyes. This is what she remembers when she finally sees him looking at Rose Tyler face to face, and no, there's no mistaking it. All those soap operas and programs, romantic comedies on the movie channels, Jane Austen adaptations; she's looked for it everywhere in life and it's not as easy to find but she knows it. Donna Noble knows the look of someone in love. And something that had been so hard to imagine before seems like the most natural thing in the universe. She doesn't remember how destroyed he looks later, back in the TARDIS, leaving the bay in Norway. Donna's too busy with mountains that move in the breeze and the easy fix for the long-broken chameleon circuit. She's too busy burning up to realize those mournful eyes are back, worse now-turned toward her-and that now he's the god of two more lost things.
Later on, when she can't remember their faces or who they are, she still remembers in an abstract, vague sort of way. She compares every set of lovers on the telly with the set she's she's made up in her mind: a tall, skinny bloke with wild hair running full-tilt toward a grinning, doe-eyed blonde down an abandoned street. He runs and runs in her mind, long strides, breathless like he's waited all the long hours of his life for this moment. It's a story, she knows, something about a god and his lover, maybe she it saw at the cinema when she was a kid but it's her paragon. She invents an embrace at the end of it, something incredible and gorgeous-something they deserve-because, like a lot of things these days, she just can't remember how it ends.
