Via Engineering Happiness: A New Approach for Building a Joyful Life :

The sociologist Ruut Veenhoven and his team have collected happiness data from ninety-one countries, representing two-thirds of the world's population. He has concluded that Denmark is home to the happiest people in the world, with Switzerland close behind... Interestingly enough, one of the more detailed points of the research found that 92 percent of the people in Denmark are members of some sort of group, ranging from sports to cultural interests. To avoid loneliness, we must seek active social lives, maintain friendships, and enjoy stable relationships.