Even when a prohibition in a fairy-story is guessed to be derived from some taboo once practised long ago, it has probably been preserved in the later stages of the tale's history because of the great mythical significance of prohibition. A sense of significance may indeed have lain behind some of the taboos themselves. Thou shalt not - or else thou shalt depart beggared into endless regret. The gentlest 'nursery-tales' know it. Even Peter Rabbit was forbidden a garden, lost his blue coat, and took sick. The Locked Door stands as an eternal Temptation.
J.R.R. Tolkien, The Monsters and the Critics and other essays
fairy-tales