Prologue:
Demons don't fall in love.
They are devoid of emotion. They have lost the ability to feel.
Yet they take on human-forms that are attractive and appealing to the naked human eye. With their perfectly shaped bodies and their inhuman abilities, they make humans fall head over heels for them and offer up their hearts.
But what do the demons do with those hearts?
Why, they keep them in a jar, of course.