Love, he thought, is a fickle thing.
The moment that he thought this wasn't a particularly stressful or meaningful moment in the least. He was fiddling with the settings on his sonic screwdriver, sitting in the captain's seat, which he had never previously known could, in fact, hold two people. Rose was asleep (for her, it was that time- probably two or three in the morning), having dozed off watching over his shoulder as he explained why exactly his sonic device worked.
She'd begun to look drowsy, almost as if she could barely keep her head up, so he'd carefully and casually shifted to make it easier (and obvious) for her to rest her head on his shoulder. That was one of the things that he'd noticed so recently- how she was the perfect height. Short enough not to have to bend herself over to rest her head beside his neck, tall enough for him to be able to look into her eyes without really having to look down. The other thing that he'd noticed was how effortlessly their hands seem to find each other, and how perfectly her body fit with his when he had the privilege to hold her.
Had the Doctor been in love before? Definitely. As much as he hated to admit it, the entirety of his prescience in the Time War stemmed from that. From so desperately wanting to prove himself to a time-lady. He knew what love was, had felt it for many if not all of his companions. He'd loved Sarah Jane, and Susan, too. He'd loved Captain Jack in his own way, like an annoying younger brother. He'd have taken a bullet for any one of them, and in some cases, he had.
But the way he felt towards Rose was something entirely different. When she looked at him, a large amount of his blood seemed to rush downward. His brain often stopped functioning entirely normally when they had a moment to just… speak. The way she made him feel was something he wasn't really sure he was ready to feel quite yet, not for anybody. And the worst part was how much she trusted him, how much she had come to love the life he lived. He knew time was running out before he'd do something stupid, and she needed to be set free- all humans did eventually. None of them can live forever and it's not fair, he thought, to make them live their lives with him, even if that's what they think they want.
Because he knows from experience that none of them have ever anticipated how long 'forever' really is. After a few years, if they're really devoted, they start getting tired. Dull. Easily annoyed. They stop loving life like they did, and then, even if he returns them home, they'll never take nearly as much joy in what they do, because he's ruined it by showing that even with the whole entire universe at your fingertips, all of time and space- it still gets old, gets tiring. And nobody can love him enough to be able to face that reality. Nobody can ever love him enough to be able to understand that it's quite possible that mortality is a blessing and not a flaw.
If anybody could fit that bill, it would be Rose. And with every aching part of his two hearts, he wanted her to stay with him.
But more of him knew that keeping her near would break her spirit, and the only thing worse than having Rose by his side would be killing that sparkle in her eyes that kept him daydreaming forever instead of working on the TARDIS, like he should have been doing.
He cupped her head with one arm as he slid out of the chair, then scooped her into his arms, smiling at how easily her head nested into his chest, and hating himself for letting even that simple gesture flood him with so much emotion.
As he had done countless times before, he walked as smoothly as he could down the hallways of the TARDIS, following the faint green lights that led to Rose's bedroom.
He laid her carefully on the downy-feather mattress, pulled the comforter up to her chin, and stood for a few moments, just watching her. Watching until she shifted, rolled onto her right side (as she always did), and stretched out one arm, leaving her hand open slightly, as if awaiting something.
It was the same way she always slept, he knew.
All the same, he didn't turn away and return to the control console. This time, he reached out and touched his fingers lightly with hers.
Then, finally, he turned and closed the door behind him- even though he wanted to stay.
Love, he knew, is a fickle thing. Love changes and grows and waxes and wanes like a moon. And love can make anyone- even the Doctor- do stupid, pointless things- like sit outside her door for the next four hours until she awoke.