If you really read the fairy-tales, you will observe that one idea runs from one end of them to the other--the idea that peace and happiness can only exist on some condition. This idea, which is the core of ethics, is the core of the nursery-tales. The whole happiness of fairyland hangs upon a thread, upon one thread. Cinderella may have a dress woven on supernatural looms and blazing with unearthly brilliance; but she must be back when the clock strikes twelve. The king may invite fairies to the christening, but he must invite all the fairies or frightful results will follow. Bluebeard's wife may open all doors but one. A promise is broken to a cat, and the whole world goes wrong. A promise is broken to a yellow dwarf, and the whole world goes wrong. A girl may be the bride of the God of Love himself if she never tries to see him; she sees him, and he vanishes away. A girl is given a box on condition she does not open it; she opens it, and all the evils of this world rush out at her. A man and woman are put in a garden on condition that they do not eat one fruit: they eat it, and lose their joy in all the fruits of the earth.
by G. K. Chesterton in All Things Considered