Why Does 'This Is Us' Falter with the Big Three's Romantic Relationships?

There's a lot of work to be done with Kevin, Kate and Randall and their relationships with each other, but they can't even catch a break with their partners. Somehow, Jack and Rebecca's relationship was so steady, secure and strong when they were 37, but Kate, Kevin and Randall's relationships don't work as well. Why is this? After all, Jack and Rebecca have one of the best relationships on television, so the writers should be capable of creating decent love stories for their children, too. This Is Us season 3 episode 16 basically blew up one relationship while showing what should've been one of the biggest moments for another.