The Great $20 Experiment ... Those who spent the money on others were overwhelmingly happier.

Along the same line: Ever read Viktor Frankl? Nazi camp survivor and psychiatrist, founder of his own school of psychotherapy? From own experience he found that the ones for whom life had meaning and purpose were the ones that could - mentally - survive the concentration camp. The ones on the other hand who did not find meaning and purpose in their lifes gave themselves up. He concluded that there is a deep-seated NEED in people to see "Meaning and Purpose" in their life, and that mental health issues often are caused by the lack of it.

Along the same line: The most brillant girl in my school is a leading cardiologist and head of the cardiology department of a Munich hospital. Her conclusion after decades of helping and healing people and many others doing so too: "The best recipe for finding meaning and purpose in life is to do help people. This in turn leads to satisfaction with and happiness in one's own life."