i don't think there's much magic to it. i think it's simply that reflexivity for some people grants them more energy than interacting with the environment does, for whatever reason. it may be a chemical thing

Jungian psychology (such as MBTI) deals with introversion/extroversion on a psychological level. one thing to keep in mind is that introversion/extroversion are independent of one's affinity for people. if there is a psychometric factor for people, it would be the so-called Feeling functions. in other words, you can commonly have introverts who are people people and extroverts who are terrible with people