This fact was forcefully brought home to me by Lars Walker's blog posting at Brandywine Books - http://brandywinebooks.net/?post_id=5003
Lewis later remarked that Tolkien disliked the book intensely, and Roger Lancelyn Green confirmed this from a meeting with Tolkien about the end of March 1949.
But if early 1949 was the critical incident, then we need to understand the background to the incident (and why it caused a rift) and also understand why the rift was not repaired.
This can, I think, be understood from studying the Chronology section of The JRR Tolkien Companion and Guide (2006) by Christina Scull and Wayne G Hammond - in conjunction with the biographical material relating to CS Lewis and Warnie Lewis's journals.
I have several points to make:
1. The rift was due to Tolkien - as will be seen...
2. The background was Tolkien having substantively finished writing The Lord of the Rings (LotR) sometime between 14 August and 14 Septenber 1948, on vacation at his son Michael's house in Woodcote near Oxford.
3. The rift was Narnia-induced in early 1949
4. The rift was not repaired because Tolkien withdrew to retype and revise LotR and also because Tolkien went through a psychological 'breakdown' (similar to the breakdown of 1945-6, documented elsewhere in this blog).
5. The end of the strong friendship is confirmed by the the end of The Inklings (i.e. the Thursday evening meetings) on October 27th 1949 - when Warren Lewis wrote in his journal that 'No-one turned up.'
Point 2. Finishing LotR
I believe that strong friendship between men is typically a by-product of an alliance, a joint-project.
From about 1936-1949 Tolkien and Lewis had a joint writing project - initiated by the idea of Lewis writing a Space Travel novel (which became Out of the Silent Planet and the following series) and Tolkien a Time Travel novel (which became the unfinished Lost Road and Notion Club Papers and the 'hobbit-sequel' which grew into LotR).
When Tolkien finished the first draft of LotR he did not need Lewis in the way that he had. Indeed, he now needed long periods of time alone to work on typing and refining the draft, pulling together the threads and removing inconsistencies.
Point 3 - Narnia
At a point when Tolkien no longer felt he needed the stimulus and editorial input of Lewis, but on the contrary needed to spend more time alone, Lewis revealed that he was not working along the same lines as Tolkien.
By writing LWW Lewis had (or so I infer Tolkien felt) broken-off their joint project which was initiated in 1936 - a project which was a continuation of Tolkien's long term project - dating from his his TCBS days from school, university and the army - and which might be described as a recovery of myth for modern England - a reconnecting of history with mythology intended to save the modern world from nihilistic materialism.
(In reality, Lewis was so productive, and so diverse in his output, that his writing of one type of book did not imply he had 'broken-off' the idea of writing another type of book. And Lewis's main project being non-denomenational Christian evangelism - including via the Narnia tales - does not imply that he would have stopped working on the long term joint project with Tolkien. So this interpretation of LWW would have been a mistaken inference on the part of Tolkien - if, as I am assuming, this was the reason for Tolkien's response to Narnia.)
Point 4 - Tolkien's state of mind
Reading through the Chronology for 1948-9, it is clear that Tolkien was again going through a disturbed period of psychological turmoil. From Feb 12 1948 he has three weeks leave of absence from work at the University, and goes to Brighton with his son Christopher - presumably to rest.
On 20 March 1948 he says in a letter that he has been unwell since October with 'poisonous' teeth accounting for some of the problem (presumably, a chronic dental infection - which might indeed produce long term symptoms of fatigue, demotivation and a depressed mood).
Yet, more than a year later, in a letter of 12 May 1949, Tolkien is still reporting 'indifferent health', trying to arrange two terms of leave, and has still not had the 'poisononous' teeth removed. (They were eventually extracted in March 1950.)
Then, in Autumn 1949, commences what is (for me) the single most embarrassing, and indeed disgraceful, episode of Tolkien's biography: the year-plus period of tortured maneuverings by which he tries to place The Lord of the Rings and The Silmarillion with the editor Milton Waldman at the publisher Collins - and where he chronically deceives, misleads and manipulates his long terms friends and colleagues at George, Allen and Unwin publishers.
So, the rift between Tolkien and Lewis was created and sustained by Tolkien by a coincidence of push and pull factors.
On the one hand, the pull between the two men was diminished by the completion of the Lord of the Rings - and on the other hand, the men were pushed apart by Tolkien taking offence at Lewis opening-up a new line of fictional work with the Narnia chronicles - then by Tolkien's need to work alone on revisions plus his disturbed state of mind - perhaps a depressive reaction to chronic tooth infection, and perhaps a moral lapse of yielding to the temptation of sacrificing his friendships in order the better to promote his literary works.
I think it is very likely that the rift was down to Tolkien not just for the above reasons, but because Lewis was a man incapable of taking offence or burning his bridges. While Tolkien was touchy and easily offended, Lewis retained friendships (such as Arthur Greaves and Owen Barfield) through great difficulties over many years - he also had friends of many types, both sexes, and continued to make new friends right up to the end of his life (e.g. Walter Hooper).
However, although Tolkien is, in a sense, 'to blame' for the rift with Lewis - there is also the fact that he suffered far more from the rift - in the sense that he never found a replacement for the stimulus and editorial input of Lewis.
When his alliance with Lewis dwindled, and the Inklings ended their Thursday evening meetings, then Tolkien found himself unable to complete large scale works for the rest of his life.
My impression is that, psychologically, Tolkien's alliances shifted from Lewis to his son Christopher - although naturally the relationship was of a qualitatively different kind.
While Lewis effectively got Tolkien to finish and publish his friend's work without excessive delay, Christopher himself finished and himself published his father's work - but after his father died.
These were the two great literary relationships of JRR Tolkien's life, and beyond.