Why is asking for salary raise so difficult?

Asking for a raise? What if you get turned down? In fact, what if your boss gives you good reason for turning you down? What will that do to your self-image as a competent and respected employee? Ostensibly the subject is money, but what's really making you sweat is that your self-image is on the line.

--Douglas Stone + Bruce Patton + Sheila Heen