I was working on something darker when this popped out instead. My thanks to a poster on one of the forums who suggested the Berlin dialog – it clicked.
Spoilers through "Lets Kill Hitler" and itsy bitsy "The Wedding of River Song"
I don't own anything...
Love was a word that made the Doctor uncomfortable. He almost always avoided using it, even in situations that called for it. He went out of his way to keep it out of his thoughts, even when helping others along the path. Love was for mortals. Love was for beings who needed to name everything. He was above that. It couldn't touch him. He wouldn't let it touch him.
He wouldn't let it hurt him.
But somehow, love invaded; a quiet, invisible force tightening around his hearts, growing in strength without warning or fanfare. In time, he could no longer distinguish it from his being. It was a part of him.
Well, to be fair, it wasn't actually invisible or quiet. Looking back objectively, it was at least 5'10" in heels, and usually announced its presence with a "Hello, Sweetie." On several occasions, its arrival was predetermined by a set of time-space coordinates and a confident splash in the swimming pool.
In real time though, it was a subtle invasion, creeping into his hearts and taking up residence while he wasn't looking.
At first it was the love of the game. This beautiful woman with her surprising knowledge and tantalizing secrets. He never stopped being intrigued by her, and never stopped waiting for the next opportunity to match wits. Each adventure topped the last, and each gave him a glimpse of further mysteries to come.
Then he developed a love for flirting. He discovered that he was quite good at it; better in this incarnation than in any of his previous bodies. It felt like taking his new wheels out for a spin, each time a little faster, and each time a little further. He pushed the boundaries, and then briefly dared to take his hands off the wheel. Part of the thrill was giving her control to take him to the next level. Interesting, unexpected, good.
His love of the connection with her boiled over at Demons Run. Her absence stung and his words were harsh. He thought that by this point they understood each other; that what was important to him was important to her. But she pushed back, and brought a difficult truth with her. One that he did not yet want to hear, but that ultimately only she could deliver. Being the last of your kind can make you unable to see your true self.
And they could have called it "Puppy Love". He was giddy with excitement and anticipation. His hearts sang with joy and relief as the realization flooded him. Amy and Rory's Melody – River Song – Part Time Lord – Kissing – it was all too much... Not only would the baby be safe, but she would be Amazing! He was no longer alone. There was another being like him, and for him, and with him. And kissing him.
And then he almost used the word. The real word. Love. In his dying breath on the steps at Berlin, he went all out. Knowing the pain that she would eventually face, he whispered his forgiveness. Tell River that she is forgiven. Always and completely forgiven. It was the closest he could get. He was willing to have his life end. He knew that she was capable of saving him, but at too great a cost to herself. To use the actual word felt manipulative. To make her trade her immortality for a future relationship was a bigger promise than he was willing to make on his own.
As the regeneration energy combined and swirled around them, all doubts left his hearts. He did not have to do it alone. The love that he felt for River did not confine itself to his own being. It reached out and combined with hers through time and space to create something larger. It would never be a conventional, plain-old-time-moving-forward, 'til-death-us-do-part kind of love. What they had transcended all normal definitions.
"Love" was too small a word to describe what they had. This was the problem he had with that word.