Introverts can be formed by the environment around them, if that is either from trauma or viewing the way that their parents act and following their tendencies. 

The events that happen around a child have high impacts in their future life. Even something as small as a loud sound as a baby can cause post-traumatic stress. It may cause someone to find the need to be on their own, in a place where they are in peace and quiet, or in an environment with noise that the person can control. Yes, emotional trauma can cause a person to become a lot more introverted. Along with something as small as a loud noise more traumatic events can clearly change the way that someone acts. For example, if a child is exposed to a very aggressive person, they can grow up feeling like they need to constantly defend themselves. When a child grows up with the idea that their opinions are not worth sharing then that will translate to when they are older, making them more silent. 

Another social impact that affects someone’s need to be introverted is following the patterns of their parents. As children, we learn our basic skills and beliefs sets with the help of our parents. Our language, our food, opinions, etc. If a child grows up with mainly introverted parents and sees the pattern of them getting their energy by being alone, and taking time for themselves, they start to need that themselves. 

Being introverted develops over time and is greatly influenced by the environment around them.