Rory was a different companion, and both he and the Doctor had always known that.

All of his former companions had always had the unquenchable thirst for adventure, the recklessness found almost exclusively in humans; in a sense, that was why the Doctor liked them. They reminded him of toddlers, excited of everything and not having the self-awareness needed for common survival. The Doctor guided them, taught them, made them thirst more and more for the universe. The Doctor didn't take them along to take care of them, oh no-he took them along to have someone look at the universe and gasp, he took them along to see the wonder in their eyes, the strengthened sense of adventure, the manic grin that only he and his famous blue box could produce.

And yet, Rory didn't possess these key qualities.

Of course, he hadn't been chosen, at first, not really-he tagged along because of his bright-eyed, utterly strange (and utterly wonderful) wife. The Doctor watched Rory the first few trips. He'd known that Rory could be (and would be) different. Rory didn't jump at the thought of the danger the Doctor introduced, didn't shoot fiery glares at the Doctor when he suggested that Rory stay back. Rory hadn't belonged with Amy and the crazy old Timelord; the Doctor had secretly thought that after a few adventures, Rory would give up and go home to wait for his wife to return from the weirdest sabbatical ever taken.

And then there was that whole Roman fiasco, and the Doctor wondered if he'd misjudged the nurse-almost-doctor.

Rory returned to the TARDIS with a wisdom that had somehow always been there, and one-thousand-year-old eyes. (To be fair, he did have the memory of one thousand or so years, so he was entitled to that kind of stare.) But the way Rory stood afterwards, the way he talked and the way he looked-the Doctor felt an equal in him. Except Rory was still different, still quiet and a background character. So why did the Doctor want to impress Rory when he walked into the room? Why did he feel the need to perform his best whenever Rory was beside him?

Rory had once told the Doctor that he made everyone believe that they could be better. But the Doctor disagreed now-he made everyone believe that he should be looked up to. He took along the companions that would learn and grow and try reckless things so he could save him.

But the Doctor took along Rory so that the Doctor could learn and grow and try reckless things so that Rory could teach him how to be better.