As a species, we like binary it makes counting so much easier, if a trifle unwieldy and we especially like to be able to classify people as either A or B. Actual people, however, don’t necessarily fit well into pigeonholes:
Everyone is either an introvert or an extrovert right? Could it be that’s not true that maybe some people are somewhere in between or a little of both?
Suppose nature made you an extrovert. You like attention and love being around people. But then you go to school and the other kids reject you or even outright bully you. In time you come to feel that being a loner is safer. You discover that being alone with your own thoughts can even be pleasant. You are an introvert. But if this happened to you couldn’t you still retain some latent extrovertedness? Because it’s your nature, crave the company of others and secretly long to be the center of attention but because of your experiences never be comfortable with the attention you crave?
I can speak only for myself here, but I am very much an introvert unless I have something resembling total control of the situation, in which case I will emerge from my shell. (Those who have encountered me in person on Tour will note that I didn’t have control of those situations, but that I figured it was safe to cede it for the moment.)
Besides, there’s that whole lonely-nights thing, and once you get past a few thousand of them, you start assuming that it’s the default.