My boyfriend at high school summer camp in Massachusetts turned out to have played with my brother when they were five-year-old neighbors in Kansas. Some years later, as I stood forlorn in Honolulu after my husband boarded his flight for Vietnam, a young man in Air Force uniform called my name; our families were shirttail relatives who visited in Pittsburgh when he and I were kids. In real life, coincidences happen.
Why not in fiction, then? Because fiction isn’t like life. Stories demand a logical progression of cause and effect, leading to climax and resolution. Real life is less shapely. A lot of life just happens, including the occasional odd happenstance we wouldn’t tolerate from an author.
This guideline comes from Emma Coates in Pixar’s 22 Rules of Storytelling: “Coincidences that get characters into trouble are great. Coincidences that get them out of it are cheating.”
I'm a historian who writes novels and literary nonfiction. My home base is Madison, Wisconsin.