There are some good answers, but I wanted to speak up a little for us introverts to provide some counterpoint.
There's a big difference between being able to get along with new people and liking to meet new people. As hard as it is for extroverts to believe, introverts are more comfortable alone. When we want to unwind, we seek solitude rather than a night on the town. That doesn't mean we don't have a small group of close friends, or never want to be social, just that it expends mental energy for us to be around others rather than being rejuvenating.
It's not healthy to take to extremes, but everyone has a different balance point. If an extrovert has 5 free evenings, maybe he will want to spend 4 with other people and one at home by himself. An introvert may have the reverse preference, but they both have a good balance. By all means, encourage a child who never has any social interactions to do so, but don't worry if he goes a couple weeks in between, or prefers one-on-one or very small gatherings. You don't have to enjoy working a crowd to be happy and have friends.