In his eyes, she is perfect. And he, well, he's not.
She is bright and smiling, laughter and love. He is sadness and anger, dark eyes and a darker past. Despite everything, though, his hearts could not love her any more than they already do, and he knows that he will do anything to keep his pink-and-yellow human safe from any pain or harm. Even if it requires destorying the rest of the universe to do it. But if it would make her happier to leave him, of course he would let her, though it would kill him, possibly literally. And he will always do his best to keep that glorious smile firmly in place, no matter what it takes.
Because, honestly, she brings out the best in him. And he never wants to let it - her - go.
Post-Christmas Invasion, pre-Doomsday. Inspired by "Perfect" by Hedley. I do not own Doctor Who or any of it's characters, no matter how much I want to - that right goes to BBC.