Aristotle's interested in finding out what the highest human good might be, so he sets about trying to describe the general characteristics of this good.
The truly best good would be something that we work and wish for as an end in itself…not part of an unending cycle of desire and attainment.
Knowing this much is a good start, but Aristotle wants to make sure that we don't waste any time on our quest for happiness—so he'll continue outlining what this "best good" is.
By his own definition, the best good would have to be the most architectonic: whatever it is that governs all knowledge and provides a way of thinking about all things.
Aristotle says that the "political art" is just the science (or branch of knowledge) that fits the bill.
Politics is the overriding category of knowledge that rules everything: it determines what citizens must and must not do, and it decides what people have to learn and do.
Also, it encompasses all the other noble arts, including things like generalship and household management (his two prime examples from Chapter 1).
Since the ends of the political art include the ends of these other communal roles, it must be the highest human good.
He introduces the concept of a city and its sovereignty: while the individual is important, the city (and the nation) is so much more worthy.
Aristotle tells us that this inquiry into what brings happiness to human beings is essentially a political inquiry.