Reciprocity is key. Years ago, fresh out of film school, I landed my first job, at a literary agency. I became what I thought was friends with another assistant, who worked, as I did, for an infamously bad-tempered agent. We ate lunch together almost every day. Our camaraderie was fierce, like that of soldiers during wartime. Then she found a new job working for a publicist down the street. We still met for lunch once a week. In lieu of complaining about our bosses, I told her about my concerns that I wasn’t ready to move in with my boyfriend. She listened politely, but she never divulged anything personal about her own life. Eventually our lunches petered out to once a month, before she drifted out of my life for good. I was eager to tell her my problems, but she wasn’t eager to tell me hers. The necessary reciprocity was missing, so our acquaintanceship never tipped over into friendship.