Human relationships are consistently found to be the most important correlation with human happiness.
A widely publicized study from 2008 in the British Medical Journal reported that happiness in social networks may spread from person to person. Researchers followed nearly 5000 individuals for 20 years in the long-standing Framingham Heart Study and found clusters of happiness and unhappiness that spread up to 3 degrees of separation on average. Happiness tended to spread through close relationships like friends, siblings, spouses, and next-door neighbors, and the researchers reported that happiness spread more consistently than unhappiness through the network. Moreover, the structure of the social network appeared to have an impact on happiness, as people who were very central (with many friends, and friends of friends) were significantly more likely to be happy than those on the periphery of the network. Overall, the results suggest that happiness might spread through a population like a virus.