I think personality is essentially a decision we make on how to deal with life. As such, I think our personality emerges gradually from basic rules we choose as children, which have larger implications and mold who we are.
For example, I think some people at some point decide "being nice is important to me". I think other people decide that "being smart is important to me". Our personalities and predispositions may follow from such essential choices.
My question for you is: what is the central rule that dictates how you live?