Shyness

[Vitruvian]

its the chemicals in your brain. dopamine/serotonin etc (there are more) the way these chemicals interact give personality.

If you think of personality, you could think of it in two ways.

Temperament : biologically present at birth or at least developed at early childhood.
Character : personal attributes developed over time due to social, economic and life experience.

As the majority of people would have a reasonably stable and balanced brain chemical level, I would think the latter would have greater influence.

Then again..