I’m not sure what you mean by “sharing a friend,” whether you’re talking about two people sharing their friendship with each other or one person sharing a second person with a third. Regardless I love the characterization of a friendship as a “spiritual child” (as per Loraine Halfen Zephyr, not Lisa Miller), an entity that two share which is unique in spirit and experience. Any two friends no matter how close or casual have at least one moment that is unique to them and that each can recollect as a unique and bonding moment. On another subject I’ve been musing on the importance of honesty in a relationship, especially honesty that damages a friend’s vanities. Maybe the “profound courtesy” you mention is the sparing of a friend’s feelings?