Out of This Perpetual Dog-Fight

Christian History & Biography has an article about C.S. Lewis and the influence of friendship:

"You modify one another's thought; out of this perpetual dog-fight a community of mind and a deep affection emerge." That is what C.S. Lewis said about his enduring friendship with Owen Barfield, who greatly influenced his bedrock views on imagination and myth.
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Rather like the texts of literature, a friend provides another vantage point from which to view the world. For Lewis, his different friends opened up reality in varying ways. Owen Barfield, for instance, was very different from Arthur Greeves, who had revealed to Lewis he was not alone in the world. Though Barfield shared with Lewis a view of what was important, and asked strikingly similar questions, the conclusions he came to usually differed radically from those of Lewis. Throughout the 1920s, the two had waged what Lewis called a "Great War," a long dispute over the kind of knowledge that imagination can give. As Lewis put it, it was as if Barfield spoke his language but mispronounced it.

Lewis's friendship with J.R.R. Tolkien, like that with Barfield, was based upon irreducible differences as well as likenesses. Initially, the two were drawn together by a love of myth, fairytale, and saga, a bond that deepened when Lewis became a Christian. There were emerging differences of temperament, churchmanship, and storytelling style, however, which strained yet enriched the friendship.

link to the entire article by Colin Duriez, "The Way of Friendship" at Christian History & Biography