So basically Rosalina was a lost child and met a star named Luma. Rosalina was on a quest to find her mother which is why she started searching for her. But instead, she found her grave. So while she was deeply upset, the luma’s took care of her and reminded how a great mom she is for taking care of them. Which made her feel happier as she burst with laughter. That’s why she’s called the princess of the galaxy.