There are many different reasons why one may have a difficult time socially, including one's temperament, culture, upbringing, and environment. Sure, some people are simply shy and truly enjoy spending time alone, or with a select few. Others, however, may be truly unable to see the world from the perspective of others, and find it difficult, if not impossible to relate to other's points of view. These may be the very same people who tend to think in a largely logical manner, and find it very difficult to relate to those who think (or feel) on a largely emotional level.
Furthermore, social anxiety may play a big role in the avoidance of social situations. Those who have social anxiety often fear the possibility of somehow being judged negatively by others. These folks often shy away from any situations which may open them up to judgment, foreclosing on any possibility of positive social interactions. Additionally, those who may have been deeply hurt by others in the past may simply choose to avoid any close social contacts in order to avoid repeated injury.
Clearly there are a number of reasons why people may find socializing with others to be problematic. Depending on the reason for the social deficit, however, some social skills can be explained, taught and practiced. For some people, behavioral interventions and practicing the give and take of social discourse, both individually and in a group setting, can be very helpful. For others, working through their past experiences by understanding and accepting themselves and their emotions may help free them up to have less guarded and more fulfilling interactions with others.