Why are human societies unstable?

Why are human societies unstable?

In the past our societies were top heavy. Whenever a change was called for, like death or defeat of the ruler in war, the settlement found itself shattered to the core. Lack of stability was writ large on our ancient cultures.

Now there is no weight on top. Each part of the bottom goes on it's own way, the heavier parts make a stable group, and the lighter ones, idle. Though the idle ones  remain poor, others extend a helping hand. And there is every reason for the world to be stable.

But it is not so. In fact it is costly to be poor. People don't realize this as the cost is quietly borne by everyone in the society, except the ones permitted to be poor. Building the necessary infrastructure to enable the poor to lead a life, that too of an acceptable living standard, and constantly monitoring all related aspects, adds to society's load. It is the unequal distribution of this load that makes a society unstable.