Here, Tolkien gets to the root of why fairy stories are compelling:
…at no time can I remember that the enjoyment of a story was dependent on belief that such things could happen, or had happened, in “real life.” Fairy-stories were plainly not primarily concerned with possibility, but with desirability. If they awakened desire, satisfying it while often whetting it unbearably, they succeeded.
-J.R.R. Tolkien, On Fairy-Stories, (Children)
This is in line with C.S. Lewis’s assertion that Joy = desire. These “true” mythologies give us a glimpse of something deeper and greater. As revealed at the end of Lewis’s autobiography, they are signposts to our creator. A good fairy-story is chock-full of these signs.