Many successful people at the top of their field hit a certain age, and the excitement of work starts to diminish. It may seem counterintuitive: Wouldn't mastery lead to satisfaction? Social scientist and author Arthur C. Brooks thinks of it another way: That there are two S Curves of life. If you're starting to feel restless (especially after age 39), perhaps it's time to stop fighting the first curve, and embrace the second. His latest book, "From Strength to Strength: Finding Success, Happiness and Deep Purpose in the Second Half of Life," goes in depth on how to reap the well-being you've spent your early career cultivating. Overcoming "success addiction" and resisting the urge to constantly add things is a big part of the equation.