Well some people are shy for many reasons - low confidence, lack of social skills, can't relate to others, social anxiety, fear, becoming nervous, anxious of meeting new people,and many more. You can put them in a group and say they're all introverts, but really we're all human and that label doesn't tell us apart. Some people are more outgoing with others because they're a people person and they know how to react to situations versus someone who isn't exposed to much attention. Everyone has different personalities and some are more shyer than others. Whether you've been brought up to be more social in situations or being isolated from social interaction, it varies. Some just keep a close range of friends, others are comfortable in large groups of friends or rather be alone. It comes down to whether or not you're hesitant to act or react in a manner suitable to whom you meet.