If you don't think you can live up to other people's expectations then it makes sense you'd try to protect yourself by minimizing expectations. So you decide that it's better to play small than to risk humiliation. Plus, you get extra points for being modest -- especially if you're female.
According to Joyce Gioia, Strategic Business Futurist, at least 54 percent of today's employees are ready to change jobs. I have read that the percentage of top performers eagerly waiting to leap is even higher.