I think I've always felt the same. Even if someone pressured. To be a certain way, I would become unhappy and revert back to my original self; much to the dismay and astonishment of my peers. It could have been I was simply stubborn.
Mental notes based on observation helped me learn who people were and how to interact with them. I noticed behavioral traits, facial expressions, dialogue, and started to differentiate between lies and half truths. However, my intuition about people didn't fully develop until high school. That is when I began to "tune in" to the emotions of people around me.
I'm sure other INFJs have had different experiences depending on their own maturity, cognitive developments, and outside influences. I felt like I was forced to understand people better partly because of my own personal experience which stemmed from difficulties with my health, and having a somewhat unsupportive home environment along with an absent father.
I do believe that nurture can be impactful in anyone's life; whether it be good or bad. Maybe it will manifest itself in such a way that it toughens you up, or weakens you.
Say if someone were part of an abusive relationship during their childhood. They may grow up to despise the world. Those who had too many things as a kid and were spoiled may become selfish.
That said, I don't believe nurture dictates nature. I suppose I believe everyone is born with a set personality, but nurture, or lack there of, decides which way that personality will go. If that makes sense.