In a broad sense, politics include several topics that every human society has to address:
Who is in charge? Who makes the final decision in matters of interest to all its members?
How to settle disagreements and disputes between its members?
Which are the rules / laws that govern society?
As we can infer from these questions, politics is concerned with two major issues: cooperation and conflict. Throughout history, different societies have come out with different solutions to promote cooperation among its members and to solve internal and external conflicts. The survival and prosperity of a society depends crucially on the adequacy of its solutions to these challenges.
Naturalism versus contractualism
We are a social species. There are several innate pro-social characteristics in all of us. Very few adult people choose to live completely alone, and even they have gone through a socialization process.
Because of the natural roots of our social behaviour, many thinkers have proposed that political institutions are also a natural development for us: humans tend by nature to create institutions that promote cooperation and sove conflict. It is true that the specifics of each society are different, but all human groups end up creating a form of government simply because it is in our nature to do so. That was the position of Aristotle and many others, and it is called political naturalism. Aristotle in particular thought that the Ancient Greek city (the polis) was the natural culmination of political development: all human groups ended up creating autonomous cities if the circumstances allowed in very much the same way that bees build beehives if the right conditions take place.
By the time Aristotle defended the natural origins of political institutions, another group of Ancient Greek thinkers (the so-called sophists) defended the opposite view: political institutions are the result of human tradition, agreement or power struggle between social groups. In summary, politics is not natural but conventional. One particular type of conventionalism is contractualism. Contractualist thinkers sustain that governments are created by agreement. If an agreement is not reached, individuals are perfectly happy living in families or small communities. Individuals are driven to create and maintain political institutions not because of a natural political impulse but by other motives such as avoiding dangers or obtaining better living conditions.