I believe that each soul has a unique base personality that they are "programmed" with. Beyond that, experiences do have an effect. You might just be naturally of a shy personality type, or you may have learned not to be outgoing.
Looking at the constants in my various lifetimes, I seem to have a basic personality trait of being outgoing and honest to the point of social recklessness. This is just the way I acted on a regular basis. However, as one might imagine, this has caused me a lot of strife in past lives (particularly in positions of power or fame). In this lifetime, this behavior pattern, which feels like an integral piece of my identity, only appears in situations where I feel completely at ease (usually only with close friends). Other than that, I have become rather socially cautious. Since childhood, I have had this pressing need to be politically correct, backed up by intense fear when I realize I have made a social blunder. I am positive that this is a direct effect of past life trauma, based on how I know I used to act and think and certain events which transpired because of it.