They want to identify with the success of their profession and their organization. They want to contribute to giving society more comfort, better health,, more excitement. And their greatest reward is receiving acknowledgment that they did contribute to making something meaningful happen. In some working environments, people are doing jobs they don't enjoy for managers who never show their appreciation. These employees conclude that if this is what it's like to work here, at least they ought to be paid well.
Money thus becomes a psychological exchange for enduring a miserable job.
Unintentionally, these managers implicitly send the message to employees that unless you get cash, your contribution to the company isn't important. Essentially, they train employees to expect cash as the only true form of thanks. It's true some people directly correlate the amount of money they earn with their perceived worth to the organization. You need to be careful, however, that you don't just respond to those individuals who constantly ask for more money, since you need to reinforce results, not requests. Also realize that you will never get the best effort from employees just by paying them more.