A truism begins our analysis - one cannot be rich and poor at the same time. And wealth is a very important factor, one of the major causes of faction and revolution, as we see in Bk V, and by considering history. Aristotle recommends a middle class, those with a moderate and adequate amount of property. The middle class is a mean between two extremes and serves as a natural basis for polity or mixed regime. Compare those of the middle class with those who stand in excess or defect with respect to wealth, strength, nobility, connections.
Extremes become unreasonable, while the mean is reasonable.
Finally the middle class can encourage friendship and cooperation. Politics needs such friendship and unanimity -rule of equals and peers. These factors upset political stability, and ultimately do away with political freedom as well. That is, they tend to tyranny. Middle class can temper the extremism.