In 1938, Harvard researchers embarked on a decades-long study to find out: What makes us happy in life?The researchers gathered health records from 724 participants from all over the world and asked detailed questions about their lives at two-year intervals.Contrary to what you might think, it's not career achievement, money, exercise, or a healthy diet. The most consistent finding we've learned through 85 years of study is: Positive relationships keep us happier, healthier, and help us live longer. Period.The No. 1 key to a happy life: 'Social fitness'Relationships affect us physically. Ever notice the invigoration you feel when you believe...