J. R. R. said it more than once, that TLOTR was Chris's story: his dad wrote it for him. He sent whole completed chapters to Chris where he was stationed in S. Africa with a Hurricane squadron. Had Chris not been a young man in need of a break in the monotony, his dad would probably not have felt so motivated then to write.

And one really important factor in getting J. R. R. to keep writing was his friendship with C. S. Lewis: the professor read every word to Lewis to solicit his opinion. And Lewis was agog, and close to tears at times, listening to his friend read TLOTR to him.

Lewis tried his own "brand" of fantasy later with "The Chronicles of Narnia", which J. R. R. did not like very well: so the favor of an appreciative audience was not reciprocated. (I have wondered how that went down between them: did Lewis feel any hurt over "Tollers'" criticism of Narnia ?)