"He who is unable to live in society, or who has no need because he is self-sufficient, must either be a beast or a god." (Aristotle)
Aristotle's Politics is said to be a natural follow up to Nicomachean Ethics. In this book he outlines what it means for men to live in society, follow laws and accept a hierarchy of rulers and the ruled (or slaves and masters).
Continuing, Aristotle says that in all relationships there is a master and a slave. In marriage, the man is the master over his children and wife. In society, people must obey the ruler. One of the best forms of government is an aristocracy, where the naturally superior rule over the inferior. One of the worst may be democracy, where everyone has a voice in the rule, even though many should not. We would find that shocking today, although our country's forefathers were aware that the unwashed masses could be easily influenced by a persuasive demagogue. Also the common folk do not always make informed decisions - which is something that Aristotle predicted (and probably witnessed).
Aristotle rails against the creation of currency and interest on money, which he considers unnatural. When people barter, they only take what they need. Likewise, the bartering tools serve functions (shoes, food, animals) while coins are only money and are not good for anything else. In addition, currency can be accumulated and this encourages greed. Interestingly, Aristotle says that the best way to make money is to get a monopoly. He lists examples such as those who cornered the iron ore market or had control of all the olive presses for making oil. It is clear that Karl Marx would have approved of some of Aristotle's beliefs.
Why do we have government? Aristotle thought the development of the state was natural for man, since man is a political animal. Starting with bonds between a man and woman, to a group of families (a village) to the much larger city-state, just as men and women need each other for companionship and procreation, a state develops out of a need. This need is the "good life". Within a state man is the most perfect of animals, but separated from law and justice man is at his worst.