For a long time, researchers have known that physical activity increases ... of life and prevents depression. But the worried was that they didn't know exactly why. Now, however, we have the answer: exercise increases levels of phenylethylamine. This is a chemical in your body that puts you in a good mood and makes you feel more ... . Whether your exercise is something you do alone , like perhaps jogging, or a more ... team sport, like rugby, phenylethylamine will make you happy. So in the ... , doctors can be more confident when they tell their optimistic patients to start exercising, rather than giving them pills.