Robin Dunbar on Friendship

 From Robin Dunbar's book Friends .  

Why Do Friendships Fracture?

One of the seminal studies of relationships and and relationship breakdown was undertaken by the legendary British social psychologist Michael Argyle (with whom, in the 1960s, I had classes as a student). During the 1980s, he and his collaborator Monika Henderson ran an extended series of experimental studies examining the rules that underpin friendships. They identified six rules which were essential for maintaining a stable relationship. They identified: standing up for the friend in their absence; sharing important news with the friend; providing emotional support when needed; trusting and confiding in each other; volunteering help when it is required; and making an effort to make the other person happy. Breaking any of these rules, they suggested, was likely to weaken the relationship, and breaking many was likely to lead to complete relationship breakdown.

Two points.  This is particularly useful in viewing ourselves when we know that the other person broke three of these, which prevents us from noticing the one we broke. Second, there is nothing in here about who did what first.