"Happiness cannot, like a mathematical theorem, be proven," wrote German poet Heinrich von Kleist to his sister in 1799. "It must be felt if it is to exist." However, over the past decades, researchers have tried to do just that, looking at what it means to feel happy and what can help us to achieve that state. According to Dorothee Salchow, joy derived from luck, like winning the lottery, doesn't make us happy in the long run. Instead she likens happiness to a state of "well-being, the inner experience of human happiness." Salchow, a life coach trained in positive psychology, knows what leads...