
People out here criticizing Leo but as his friend said, their species is weak to love and how they "blindly cling to it when they fall in love" especially if they are human. We don't know the full story but we do know something happened with Leo's uncle and aunt because of love, where his uncle was possibly human. It is possible that once they fall in love they become obsessed with the other party, which is why Leo prepared a contract for Sungwoo. It's not like Leo ever treated Sungwoo badly, and is being pretty considerate to him.
Poor disciple, knows too much of his dark history so he never wants to meet her again xD