Friendship is most difficult because it does not in fact last, at least not in the way we thought it would, swore it would, when we were younger. Aristotle already noted this in his Ethics, but he is hard to believe when we are young. We change. Our fiends change. Friends must grow proportionately together, though perhaps not in the same ways. And when we or our friends take certain options, certain choices, we can no longer abide with them, since friendship is primarily a sharing of truth and goodness. The ancients were mostly right. I suspect that if by the time we reach fifty we have two or three friends, we are lucky.