He is the Immortal Lord and she is the Demon Lord. Each is the master of his own realm, so why are they entangled?
He went through countless hardships in the lower world for her, and she was also brought down to the mortal world. A hundred lifetimes later, he was still planning for her! I'll do everything for her!
She is his defeated general, and she is not willing to accept it! Setting up the battle with love, not only he but also herself entered the battle.
She is also free and easy, and I love her when I love her. She is the demon lord derived from the breath of the earth, she can do whatever she likes!