What rules do good friends follow? (thoughts wanted)

I’ve been thinking about friendships and why some last and others fade away. It seems there is an unwritten set of rules people who stay friends are able to follow, even if they don’t even sit down and discuss them.

I’ve certainly never had a friendship where there was an official meeting, where a  friendship charter was drawn up detailing what everyone expected of each other, or planting seeds for how to deal with difficult situations that might arise. Have you ever been a friendship where a compact for how the friendship should work was discussed?

It seems strange to me how sometimes the most important relationships in our lives are assumed to not require the same investment of consideration for how they function (or fail) as the relationships we have at work.

I’m looking for your thoughts on what the implicit, or explicit, agreements friendships that last have. And any stories you might be willing to share about how you arrived at those ‘rules.’ Or how the lack of them impacted you.

Looking forward to your thoughts.