Friendship arises out of mere Companionship when two or more of the companions discover that they have in common some insight or interest or even taste which the others do not share and which, till that moment, each believed to be his own unique treasure (or burden). The typical expression of opening Friendship would be something like, "What? You too? I thought I was the only one."
... It is when two such persons discover one another, when, whether with immense difficulties and semi-articulate fumblings or with what would seem to us amazing and elliptical speed, they share their vision - it is then that Friendship is born. And instantly they stand together in an immense solitude.
Life Quotes And Sayings by C.S. Lewis, The Four Loves