What is it that drives some of us to love trying absolutely crazy stuff, like the base jumpers?

I know it’s nuts, but I’ve done some crazy stuff in life and taken some stupid chances. Why do some of us thrive on that rush? And I have promised not to do it anymore, so it’s not happening in the future. But why would base jumping, auto racing, or other crazy stuff have such an allure. What gets us off on pushing the limits?