Many people have been conditioned that one must always be around people. Our society is an extroverted one and thus emphasizes, even glorifies socializing, mingling, and any activity which centers being around people. People who are more comfortable being alone are viewed as suspect, even abnormal. Being alone and not preferring the company of people is oftentimes demonized in our extroverted culture.
Children and people who are more introverted are pressured to mingle-to get involved with activities although they are not interested in such activities, even preferring more solitary pursuits. Many times people want to be galvanizing because they are fearful of introspection. People really hate to be alone with themselves. They may even consider such to be selfish, even wasteful. They would rather engage in mindless social activities than to spend more constructive time in solitude, developing themselves emotionally, mentally, and psychologically.
Then it is a familial thing. Our sibling society and culture emphasizes socializing and mingling. In multichild, particularly large/very large families, children are pressured to be part of the group while individual pursuits and talents are discouraged. That explains why people from large/very large families particularly DO NOT like being alone and MUST be with people. It seems that people from large/very large families have a phobia of being alone.
Only children are encouraged to be alone. In fact, only children view being alone as a way of life. They feel that they can think, read, and be introspective. They also feel more creative when they are alone. Children from small families (1-2 children per family) are encouraged to have private and me time for parents of such families realize how important that privacy and me time have in a person's development. This results in people from small families feeling very comfortable being by themselves and pursuing solitary activities and hobbies.