It's a fitting metaphor for the way relationships work. We all encounter people who remain in our lives only for a specific period of time, whether because we work with them, socialize with them, or enter into romantic relationships with them that do no become permanent. Is it random? Happenstance? Strout plays with that theory at several points in the book, as have philosophers for many centuries. I think it was particularly apt vis a vis the lockdowns during the pandemic when normal relationship patterns were disrupted and for so many people the future felt tenuous, uncertain, and maybe even too frightening to contemplate. Who would we collide with next? For how long? Would those persons survive? Would we? Without getting overtly politicial in her narrative, the author makes several references to the individual who held the Oval Office hostage during the pandemic and utterly botched the handling of it. The chaos in D.C. contributed to the anxiety felt by so many Americans.