要看你对贪婪的定义了,贪婪不同于自私,贪婪应该理解为对自己既有的利益或是现状的不满,古人说的“欲壑难填”。
A rational explaination is the shift of reference point. People's utility depends on the deviation from reference point, not just the wealth he holds.
You can refer to Kahneman and Tversky's Prospect Theory.