“The purpose of our lives is to be happy.” The Dalai Lama – Buddhist monk and spiritual leader of the Tibetan people – points out the simple and universal truth that all of us naturally gravitate towards happiness. We’re on a constant search for that joie de vivre – or “joy of living” – in our day-to-day, whether it be a joy found in love, companionship, work, or hobby. Then, there are countless motivational or “self-help” books published on the topic of positive thinking and finding happiness within.  But, what about those cloudy days when we don’t feel very joyful? What’s happening in our mind and body that’s affecting our mood? Serotonin, the “happy…