Why do the highest paid people complain the most?

After managing over a dozen people I have noticed a pattern. The people who are paid the most tend to be the least satisfied and make the most problems. Currently I am working with a small group of 4 people. 3 of them are paid $20/widget and the 4th is paid $30/widget. The 3 lower paid people do their job well and on time and I rarely hear from them. But the 4th one is regularly pestering me asking for a raise or a loan or an assistant. It's exhausting.

I have faced similar situations win the past. Has anyone else noticed this?