March 17 was the finale for the fourth season of the NBC show, “This Is Us.” Set in Pittsburgh, the story follows three seemingly unconnected young adults who share a birthday. “The average human being,” we’re told, “shares his or her birthday with more than 18 million people.” In the pilot we see that Kate, a morbidly obese personal assistant, celebrates her 36th birthday by recommitting to losing weight. Kevin is an actor famous for starring on a popular sitcom. On his 36th