At first, he doesn't believe her. He doesn't believe her promise of forever.
She's only twenty when she makes the promise, with their hands swinging in between them and surrounded by flying creatures. He doesn't say anything to her, doesn't mention that there's no possible way she can keep that promise. He just forces a smile and gently leads her back to the TARDIS.
She's only twenty now, but she'll age. It may be a while (for her) until she's thirty, until she's forty, but the years will pass by quickly for the Doctor. In the blink of an eye, she'll be older, and she'll want to stop traveling with him. She'll want kids and a nice husband, a nice steady life.
Which is the one thing he can't give her. So she'll leave.
He had known this moment was coming. The moment when they'd have to say goodbye, forever. It always came.
But she wasn't ready to leave. And he wasn't ready for her to leave.
She was still twenty, still ready to give him her forever. She was still thirsting for adventure, for the thrill of running from hostile aliens and the feel of traveling across time and space with him.
Her stuff was still thrown everywhere around the TARDIS, her room the exact same as it had been when she'd left earlier that day, still expecting to come back and go on that trip to Barcelona he had promised her. And then she's gone, trapped in a parallel world, and he can't bring himself to pick her stuff up, still holding onto the impossible hope that this was just a big joke, a dream. But it's not, and there's no way for him to get her back.
He finds a crack in the walls, a way for him to say goodbye. It comes out in Norway, on Bad Wolf Bay of all places.
Rose is there. She looks the same, still twenty and still wanting to be with him, still wanting to travel the universe and see everything time and space has to offer.
There's so much he wants to tell her, but they have so little time together. He tells her that she and her mother are on the list of the dead, back home. She asks if she'll ever see him again. He tells her she can't.
She tells him she loves him.
But their time is up, and they're ripped apart again before he can finish telling her what he's been dying to say.
There's no Rose in the TARDIS, never will be again, but he still whispers the words, his soft I love you hanging in the air.
He does his best to distract himself.
He saves the world with Donna, brilliant Donna. The racnoss are gone, Donna's home safe and sound, and the Doctor is once again alone.
(He tries not to think about Rose, and how Rose would have loved Donna)
Then there's Martha, and Astrid, and Donna once again.
He doesn't make the same mistake again, he doesn't ask them how long they're gonna stay with him.
(The word forever keeps repeating in his head, in her voice. He tries not to think about the fact that he'll never hear her voice again)
She comes back to him. He shouldn't be surprised, because she's Rose Tyler and she can do anything.
They save the day and the Doctor knows he's beaming, his arm wrapped tightly around Rose. He wants to kiss her, wants to tell her that he loves her, but he wants to do that when they're alone, when everyone else has gone.
When it's just Sir Doctor and Dame Rose in their TARDIS, like it should be.
The years go by. He regenerates, into a man with floppy hair and a strange fascination with bow ties. She stays with him, keeps telling him she loves him and that she'll stay with him forever, even if he turns into a slight arsehole.
Forever.
He wants to believe her.
But she's in her thirties now, and he still feels like any day now, she'll ask him to take her back to Earth or some other planet so she can live a normal life.
She'll ask him, and he'll take her. She'll be ready to leave, and he'll let her.
But he won't ever really be ready for her to leave.
She's fifty now, but she's still with him. They can't go on their old adventures, because she can't run like that, not anymore, but that's okay with the Doctor, because she's still with him and they're okay.
She's given up dying her hair, letting it return to it's natural chocolate color. There are gray hairs, but he still thinks she's the most beautiful person in the universe.
Her promise of forever is seeming to be more and more believable, but there will always be that small sliver of doubt that one day, out of the blue, she'll want to leave.
They can't go on adventures anymore. Rose is sick, and old, and she needs a lot of help, but the Doctor still loves her. Her time is coming, they both know it, but only Rose has accepted it.
The Doctor has been taking care of her, helping her eat and shower and go to the loo. He doesn't mind, he'll always want to help her in whatever way he can.
But it's hard, watching the woman you love slowly wither away and not being able to do anything about it.
It's even harder when she starts questioning things, asking him if he wants her to leave. His answer is no, it'll always be no, but he can tell that Rose is less and less sure of that.
Sometimes, when they're sitting in the galley or relaxing in the library, she'll ask him if she's still the most beautiful girl in the universe. Her hair is all gray and she's got wrinkles, but her eyes are still big and brown and beautiful, and her smile is still as radiant as ever.
Forever, he tells her. You'll forever be the most beautiful girl in the universe to me.
She doesn't really believe him, and the Doctor knows that.
It's funny, he thinks, how their roles could be so easily reversed. She was supposed to be the one promising him forever, and he was the one who was supposed to not believe her. But now he was promising forever, and Rose, his most precious Rose, didn't believe him.
She was there one day and gone the next, passing away in her sleep at the age of 97. She had kept her promise, had given him her forever, even if her forever wasn't nearly long enough.
He hadn't been ready for her to go.
Authors Note: Mostly inspired by sad/beautiful Elvis songs, including (but not limited to) Young and Beautiful and It's Now or Never.