A study conducted by Singapore Management System and London School of Economics and Political Science explores the "Savannah Theory" that smart people like to spend more time alone than others. The Savannah Theory, also known as evolutionary legacy hypothesis, indicates that much of our adaptation occurred in rural environments and we respond and adapt much like our ancestors did in places like the Savannah.
We are designed to live in groups of around 150 people. When we get into a larger group such as a city we don't feel comfortable and we may feel a little antsy. When groups move above 150 they begin to split and create more groups that are easier to manage. Our social networks seem to follow that pattern as well where our preference for active engagement is for 150 people or less.
The study indicates that highly intelligent people like to spend time alone. The study doesn't affirmatively say that people who like to spend time with others are less intelligent or why intelligent people like to spend more time alone. It also doesn't clarify whether spending time alone is more of a by product of not relating as well to people of lower intelligence or has something more to do with entertainment in one own thoughts.
The study states that intelligent people are more goal driving than others and prefer to work alone to achieve those goals which is an evolutionary outlier. While I find this an important argument it might also depend on what those goals are. People directed at feeling good and connecting with others for their own survival is not necessarily a bad trait. It would seem a healthy balance between being with others and spending time alone would be very high intelligence when you can manage it properly to fulfill more of your goals, needs and wants.