It all started in that damned Library.
He should have known, really, he could never have married someone who wasn't just as reckless and self-sacrificing as he was. Could never have fallen in love with someone who wasn't.
And he was, you know, in love with her. He's known since the first time she really looked at him. Right before she realized he didn't know her, her eyes were so full of hope, and excitement, and so much trust, like she knew he could never let her down - and love, so, so much love, more love than he knew what to do with. And then, in an instant, as soon as the words left his mouth, all of those emotions were replaced with a sadness so deep, he felt his own hearts break. Why did he feel so connected to this woman he didn't even know? She was pleading with him, almost begging him to tell her he knew her. He almost did, just to make that awful sadness leave her eyes.
After that everything happened so fast, the next thing he knows he's waking up handcuffed to a pole and the woman with the sad eyes and the wild hair is sacrificing herself so that he can't. But when she talked about him, the him that she knew, the him that loved her so much he had given her his name, the Doctor was afraid. How could he keep seeing her, fall in love with her, marry her, all the while knowing that she was going to die because of him?
So he ran, and he pushed her away, and he tried so hard to save her from something that had already happened, telling himself he couldn't love a woman already dead. But it was too late, that hair, and those eyes, and the way she always smiled when she knew she was being clever. And then she tore time apart just so that she wouldn't have to kill him. And he knew, he had been an idiot trying to protect himself by not letting himself love her when he knew damn well it was too late for that. So he married her, on a pyramid at the end of the world, in a timeline that never existed.
After that he kept running, but this time he was running to her, instead of away. Now that he was done pretending he didn't love her, he was going to spend as much time with her as possible. He gave her his hearts on a silver platter, but he never said the words. Didn't think he had to, he figured it was obvious, in the way he looked at her, the places he took her, the things he showed her that he only ever saw with her.
So when she stood in front of him, no idea that he was there, and said with absolute conviction that he did not, and had never, loved her, he broke. How could she not have known? How could he have been married to her for over a century, and not known that she had no idea what she meant to him? He never thought he'd had to say the words. He really was an idiot. But now he had 24 years to make sure she never again doubted it even for a second. He would tell her every day, a thousand times a day until every dark mark of doubt was erased form her mind. He was having a hard enough time sending her to her death to keep his promise to her ("not one line," those words echoed through his mind every time he got the urge to beg her to never go to the Library), but there was no way in hell he was going to let her die not knowing that both of his hearts belonged to her. Always and completely.
It all started in that damned Library, but as he watched her leave while the sun rose after 24 years on Darillium, the Doctor made a promise that it would not end there.