At least they didn't fall out over a woman.
Or a job.
Good old jealousy had much to do with it.
@JH - I don't think jealousy had much to do with it - although it is often given as a reason for Tolkiens coolness. I suspect this is projecting onto Tolkien what we imagine would have been our own feelings under the circumstances. To me it seems very obvious that Tolkien loathed fame/ celebrity and found it very disturbing. Of course, there is usually some mild kind of envy over anyone who is as productive as Lewis became from about 1936 - but given how famous Lewis became during WWII, and how much he was already publishing leading up to 1948, the timing is wrong to ascribe this as a major cause in the cooling of relationships.I find no sign of coolness up until early 1949 (a few arguments, but these were apparently swiftly repaired).Of coues, once they had fallen out, Tolkien's retrospective comments on his relation sith Lewis were affected by this fact - but this was re-writing history (as the later Tolkien rewrote history when he claimed from the late 1950s that Charles Williams had been Lewis's friend, but not Tolkien's friend)- these later comments cannot be regarded as accurate.