Richard J. Davidson's recent book examines their basis and how to change them:
[T]he most salient characteristic of emotion in people is the fact that each person responds differently to life’s slings and arrows. Each of us is unique in our emotional make-up and this individuality determines why some people are resilient and others vulnerable, why some have high levels of well-being despite objective adversity while others decompensate rapidly in the response to the slightest setback.
The Dish 