The first time that he calls Sara his little princess she is a baby, the only thing that he has left of her mother. She is his whole world, because doesn't have anyone else he cares about even half as much.
He starts using it as a nickname when she gets older, being more privileged than other children of her age, but far kinder about it. She has everything she could ever want and she wants to share it with others.
She reminds him of the best things about being royalty, the strength and belief.
She will always be his little princess.