Emily Esfahani Smith explains:
Brain size generally increases with body size across the animal kingdom. Elephants have huge brains while mice have tiny ones. But humans are the great exception to this rule. Given the size of our bodies, our brains should be much smaller—but they are by far the largest in the animal kingdom relative to our body size. The question is why. Scientists have debated this question for a long time, but the research of anthropologist Robin Dunbar is fairly conclusive on this point. Dunbar has found that the strongest predictor of a species’ brain size—specifically, the size of its neocortex, the outermost layer—is the size of its social group. We have big brains in order to socialize.
This helps explain why socialization is so strongly connected to our happiness:
When economists put a price tag on our relationships, we get a concrete sense of just how valuable our social connections are—and how devastating it is when they are broken. If you volunteer at least once a week, the increase to your happiness is like moving from a yearly income of $20,000 to $75,000. If you have a friend that you see on most days, it’s like earning $100,000 more each year. Simply seeing your neighbors on a regular basis gets you $60,000 a year more. On the other hand, when you break a critical social tie—here, in the case of getting divorced—it’s like suffering a $90,000 per year decrease in your income.
The Dish 