Here's some bad news: love is fleeting. It’s not even unconditional, despite what you’ve been taught.And that giddy, passionate marriage you’re in? It’ll probably feel stale within two years.While this all sounds like cause for depression, it’s actually an exciting starting point, according to new books from two leaders in the field of positive psychology. Positive psychology is a relatively new branch within science; rather than studying dysfunction and treating illness, researchers in this area focus on investigating and promoting mental health. They look at why and how humans flourish, asking, “What’s working?” instead of “What’s troubling you?” We all know that positive emotions—such as joy, compassion, connection, resilience, and optimism—are vital to our well-being; this field of psychology studies the actual roots and roles of these emotions.Take love, which is transformative, but not in the ways we usually think, says Barbara L. Fredrickson in her new book, Love 2.0: How Our Supreme Emotion Affects Everything We Feel, Think, Do, and Become (Hudson Street Press). Fredrickson is the director of th …