This Video Makes Me Think About Peter Pan.
The biggest thing about Peter is that he suffers from maternal deprivation. His mother shut the window on him when he was small and thus, he feels abandoned. That is why I think he cried when he couldn't put his shadow on. His shadow, like mommy didn't want him.
When you think about it, a mother should be any child's first experience with femininity and it should be positive femininity. She is caring, kind and helps with their development. While Wendy could never replace Peter's real mother, she was his first real best friend and experience with positive femininity. Wendy is so important to the story and to Peter himself.