Speaking on behalf of empirical data, many humans are not content to have just enough. They always want more. Give someone a million dollars and they want more. Give them a comfortable house, and they will dream of having a better one. It's all relative, but by nature, I think humans will never be content with what they have or know. They will always seek out something more. To elaborate on the comment above about greed, agreed: This is why Communism, one form of Marxism, in favor of a single class, will never work.