Rob Horning looks at a study on friendship:

...the study suggests that attempts to rationalize friendships in advance may fail to capture what makes friendships work, which may in part be an ineffable spirit of coincidence, hanging over and enchanting everything the friends do together. And even more so, a feeling that the friendship is authentic precisely because there is no good reason for it and no calculation went into it.

Or as Montaigne put it, because it was him, because it was me. My own extended essay on friendship, with reflections on Aristotle, Emerson, Aelred of Rievaulx, Oakeshott, and Jesus, is the last third of "Love Undetectable."