Who you are has nothing to do with personality traits, you know this. Most mature people grow outside the constraints of personality traits. If that wasn't the case then all you would have to do to get to know me, and how I would respond in any given situation would be to read my MBTI. Why does someone become cynical or insecure has nothing to do with personality, it is usually life experience and usually early childhood shapes most of what is going on in our unconscious. You can tell the adult that had a cold and uncaring father that didn't teach her/him about the world, about compassion and how to face pain and suffering with courage, someone who protected him/her when they were hurt. He might have focused too much on his work, maybe he was an alcoholic, nothing too severe, but enough for the daughter or son to realize the fraud of love by an early age. It has a huge impact on who they become.