For decades, motivational speakers like Tony Robbins have told us that positive thinking can vastly improve our lives. But Barbara Ehrenreich, the writer famous for the 2001 bestseller “Nickel and Dimed: On (Not) Getting By in America,” thinks they might be wrong. Her new book is called “Bright-Sided: How The Relentless Promotion of Positive Thinking Has Undermined America,” and she tells us why she thinks positive thinking might not be all it’s cracked up to be.