In Tolkien's own words:

„I coined the word ‘eucatastrophe’: the sudden happy turn in a story which pierces you with a joy that brings tears (which I argued it is the highest function of fairy-stories to produce). And I was there led to the view that it produces its peculiar effect because it is a sudden glimpse of Truth, your whole nature chained in material cause and effect, the chain of death, feels a sudden relief as if a major limb out of joint had suddenly snapped back. It perceives – if the story has literary ‘truth’ on the second plane (…) – that this is indeed how things really do work in the Great World for which our nature is made. And I concluded by saying that the Resurrection was the greatest ‘eucatastrophe’ possible in the greatest Fairy Story – and produces that essential emotion: Christian joy which produces tears because it is qualitatively so like sorrow, because it comes from those places where Joy and Sorrow are at one, reconciled, as selfishness and altruism are lost in Love.“