The same things that make him sad are the things that make him happy. He likes to sit and think about his friends, and the way they might react to his silly new face. He likes to think about them happy. He likes to visit them when they’re children and help with homework, or bullies, or hard times. The Doctor likes to think about changing their lives for the better, but then, he gets sad — he doesn’t change lives for the better.
So then, he thinks about his box — a funny, old box. The bluest-blue box, his gorgeous old girl, and how she thinks of him. His friends, they always leave, but his box never will. His box is the one thing that he can ever be sure of, and he’ll stay with her forever, until forever ends.