It's an odd moment for him, having to realize that he's twice as old as he is. Not in body persay, but in mind. Twice as old, and possibly twice as wise (Not clever though, no. The Doctor knows how to play clever).
There's a difference between them, though, because Rory doesn't ever need someone else to remind him of who he is, what he's doing. Rory's so ridiculously blinded by love that he'd tear apart the thresholds of the universe if it meant to see Amy safe.
Perhaps it's that sentiment that the Doctor cherishes in humans, and seeks to make it grow. Because he's the Doctor, and if he can't afford such sentiments in himself, he reserves the right to foster it in others.
"And if you say goodbye properly?"
"Two universes will collapse."
"So?"
The Doctor never wonders what would have been, could have been, had he decided to take that chance. Because there's so little time for What If's in this life, and what's living if not moving foreward, forgetting? Not really growing, not anymore, because when they leave it's always just the same.
When they leave, he should be able to do it on his own.
If he was growing.
And getting better.
But he can't, and that's how he knows that he's not. And despite living in this life of no strikes and what-ifs, he knows Rory would have made the choice he couldn't afford that day.
One day on the beach, of a parallel world.
A crazy, mad, impossible choice that could have destroyed dimensions. He would have been selfish, volatile.
Human.
That is love, the Doctor knows now. He learns it from a poor man to whom no one pays any heed. He is all the things the Doctor knows he can't afford to be.