Why do we love sad songs? It’s true, we do, and it’s not just “Greensleeves” and “Yesterday.” There’s a language of melancholy worth exploring. The saddest music of all to many people is Samuel Barber’s “Adagio for Strings.” Thomas Larson is the author of “The Saddest Music Ever Written: The Story of Samuel Barber’s Adagio for Strings” and he tells Anne Strainchamps how it earns that name.