the collaborative, amicable and (ironically) emotionally connecting ending of a relationship done when emotionally mature, emotionally healthy people respectfully acknowledge their differing wants and needs and agree together within a context of mutual respect to end the aspects of their relationship which do not serve them, sometimes enabling them to part as friends with love in their hearts.
Mature people in love who find that they just aren't right for each other don't have to find some bullshit reasons to hate each other to justify ending the relationship, they can recognize what isn't working and communicate, to have a companionate breakup.
That irritating, not especially romantic state that many married couples fall into when they are no longer young and restless- basically, they're friends with benefits and wedding rings. The purpose of this is to prevent passion from interfering with raising their kids. It's a force of nature, and it is incredibly disturbing to watch as it takes place. There is no known cure except divorce.
Bill and Sheryl had been married for fifty seven years and could go for weeks without so much as kissing. Companionate romance strikes again.