It is really mainly determined by genes; how the neural system is made will play a major role in determining the personality. Of course education/enviorment will also contribute.
I had thought about this and finally found that: it is really not that a big deal. Many sucessful people are also very very shy. The key point is to find the strongest point of yourself. Being shy is not necessarily a bad thing, as long as one can find a good way show his/her strong point.
Just my 2 cents.