Any answer I give sounds vague or trite. I think that's because what matters in friendships is not so much what you talk about but what lies underneath the conversation. That undertone is what friends hear when they listen to each other. Perhaps it is the soul's voice. Answering the question is difficult because of another characteristic of friendship.
Friends just like to spend time together.
They feel comfortable and uninhibited, unjudged, encouraged when they are together. There is a comfort at almost a soul-mate level. That does not mean that family members cannot be each other's friends. Indeed they often are friends, by birth and by choice. And, of course, spouses can be good friends.