His infatuation started with curiousity.

why you where here Douma didn't care pampering you and immediately appointing you as his favorite.

That was when he decided that.. yes, you're stuck with him. Forever.

currently he's watching you as you walk through the forest with your cute little sword. he wants you to cut him. he wants you to beat him. he just wants to be sweet and pamper you as you beat his face in. the very thought makes him giddy. your such a pretty little demon slayer, even if he is a demon. he will have you. after all humans are weak.