Now that is an interesting question. Do we change, really?
God I hope so. Next time you go to your high school reunion, pray you don't hear anyone say Larry, you haven't changed in thirty years. With luck, they'll say it as a compliment about your APPEARANCE and not your personality. Because if you're the same person you were at 16, there's something deeply wrong with you.
You wouldn't know the person I was at 20. Granted, we haven't seen each other face to face in about ten years, but you definitely wouldn't know the guy I was in college.
For one thing, I didn't talk a lot, and preferred to be alone. I rarely smiled. Girls who flirted with me were annoyed at my imperturbability. It was my natural reaction to believe most people were deliberately trying to piss me off, and it showed. You would have guessed I had Asperger's or something. When I read stuff I wrote then, it spooks the crap out of me. I was extremely self-involved and I seriously could not listen to others.
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(I just deleted about three paragraphs of stuff - I'm gonna learn one of these days, more doesn't mean better.)