He and she were the most enviable couple.
He is the second prince of the demon world, and she is the little princess of the heaven.
They grow together.
He single-handedly built a dreamlike fairy tale world for her, only to have it collapse.
After returning, she was no longer the weak and innocent little princess she had been before, and she no longer needed him.
And he didn't want to let go.
The damage from the past has been done, and since it cannot be forgotten, let him re-create perfection for her.