Why are humans so greedy?

While reading the first chapter of "Animal Farm" I learned how a certain animal named Major realizes he must take a stand against his master's "tyranny" and greed. During Major's speech he tells of all the cruelty that is exemplified on the farm. Major states that "nearly the whole of the produce of our labour is stolen from us by human beings". This statement was also made "Remove Man from the scene, and the root cause of hunger and overwork is abolished forever. The last statement reminds me of young and hungry children who worked laborious hours for a couple useless pennies while their boss get so much more. Also, I am reminded of all the revolts against the government caused by slaves that were constantly treated unfairly. People from different countries "barged" in and made the natives compromise their beliefs, pay taxes, speak a different language, and change their life styles in many horrible ways. Then, someone decided to take a stand in order to receive change. I don't know why some people in this world are so greedy and like to take advantage of underprivileged people and make their lives worse than they were. What I do know lis that greed can take power over people's actions which causes many lives to be affected including theirs.