He is the Lord of the heavens. She is his deity.
He loves her. Love to the extreme.
So much so that after she returned, in order to let her wake up again, he came to the human world and fell in love with her in the future, just to let her wake up.
In later generations, she loved him to the extreme. Even after knowing his purpose, she was still devoted to her like a spring breeze.
It was only after she really woke up that he realized that he had fallen in love with the ordinary her at some point...