The Doctor wasn't one for love. At least, not in some respects. By his own standards and in his own mind he certainly wasn't.
River would disagree, of course. As would most of his former companions. They all saw him as the most loving of beings. The only person who radiated true love and true hatred and everything in between all at once.
He himself, however, didn't think that. Not for a long time anyway. For years, he was almost scared of love. It frightened him and made him nervous and flustered and almost always blush. Why he was scared he didn't know. Only that he had seen what love could do and he hadn't particularly like it.
After a long time of thinking this, he met River. He hated to accept it, but it changed him. Meeting River. The truth was, she did things to him and his heart rate that he couldn't deny. To him she was almost impossible. The one woman who could scare him and make him love her all at the same time. She could even fly the TARDIS. She was perfect, not conventionally but that was what he loved. She confused him constantly, with her teasing and her..smile. That smile that knew everything that he didn't. And, for a while, it irritated him.
No matter how mad she made him, the thing was, she cared. She was always there. When he needed help and he could tell he would be too. In the future.
His future had love in it and he was running straight for it. No matter how scared he was, he'd much rather run forwards than back...