Susan Cain prefers to poke around the less-examined corners of can-do America. In 2012 she published “Quiet: The Power of Introverts in a World That Can’t Stop Talking,” which became a phenomenon and made the congenitally less chatty among us fashionable and even cool. The 1993 Harvard Law School graduate’s new book, “Bittersweet: How Sorrow and Longing Can Make Us Whole,” has become a New York Times bestseller. The Gazette spoke with Cain about how embracing the poignancy of life can lead to creativity and connection. The interview was edited for clarity and length.