Mirrors
I feel like the people we get along best with are always, to some degree, mirrors of ourselves. My closest friends all mirror some aspect of myself, and I some aspect of them. The more you mirror, the more likely you are to be friends.
A lot of my friends don’t have much in common, aside from being generally smart in various ways, and always having something or another that they can talk your ear off about.
But when friendships end or people get hurt, the mirror breaks. Sometimes it cracks, sometimes it shatters. It almost always hurts, though. You can see the reflections in the broken mirror, but know it’ll probably never be put back together.