I don't think the dichotomy is a bad rule of thumb to help understand someone, but it's certainly not their identity. I've come to find that it's down to the person's experience with and resulting preference for crowds and individuals. It's nurture, not nature. I used to be very introverted until I started living on my own as an adult and had shit to do and wanted things in life. I've also seen extroverts that mellow…
Being in crowds of people I'm expected to interact with on a personal level drains my energy. Baseball games, concerts, and the like? No problem! Crowds of strangers don't bother me a bit. But if there's a hell, my corner of it looks like a cocktail party where I'm expected to mingle with everyone and never get a moment to myself.
Being an introvert is nothing I was raised to be. My mom's one. My dad was a natural salesman: put him in a room with 50 strangers, and 10 minutes later he was on a first name basis with them all and had plans to vacation together. They both raised me, and they both stressed the importance of good social graces. The difference is that after my dad's social butterfly antics, he was happy and fully of energy, while my mom was ready to go home and recharge for a while. I'm definitely my mom's kid on that one.
I have stuff to do. There are things I want in life. I'm doing all that successfully. But when I'm done doing them, I just want a little time to myself to exhale and relax, OK?