"Honey!" exclaimed the demon happily, closing the door behind him "I've missed you so much! And you me, haven't you?" that excited tone made you gag.

He can't help it. He may not have the slightest idea about human emotions, but he knows clearly that only you, a mere, pathetic human, could ignite his empty heart. He just wants to keep on having that feeling. What better than to lock you up all to himself?