He's been obsessed with saving people since the Time War, even before then. But these hearts have a particular weakness for children. Amy pinned it long before he know the truth: "Never interfere in the affairs of peoples or planets—unless there's children crying."
One of his older faces made a point of being childish. The two of them might have some interesting times if their timelines ever crossed. But the odds of that happening were so small. It would be a childish thing to hope for.
Children. Somehow it all came back to the children. Small, helpless against the monsters hunting them down, innocent, yet with flashes of adult insight that were all the more terrifying for the high-pitched voice the words were spoken in. The most precious things in the whole universe.
Oh, not everyone agreed. Some would say power, money, security, romance—but they were wrong. It was the children who were most important. Because they were the ones young enough to make a difference.
Children. He looks in the mirror and smiles.
If he was little shorter, he'd fit right in.