I think it's about growing up and accepting your responsibilities. Peter never wanted to grow up, but Wendy did. I think she realized that fun and games and getting away from reality is nice, but you can't just stop your life. One day, you have to grow up whether you like it or not. It's inevitable. It kind of reminds me of Tuck Everlasting, where the boy lived forever, but she didn't. She wanted to grow up and believed living young forever really living.