Politics is inherently unstable in the short term, and as seen from within and nearby, because it is the outcome of a compromise between those interests of an individual which are in conflict with the interests of others, a fairly well understood and very large set, and those interests which coincide or overlap in some part with the interests of others, a set of uncertain size and membership. Individuals necessarily approach both classes of interests in a probabilistic fashion, since they can do no better, but all their behaviour is encompassed by this description, and it is unobservant to claim otherwise. Politics is the attempt to optimise results in both categories.
The instability arises because:
1. Interest, both purely individual, and that overlapping with the interests of others, is uncertain and dynamic. The boundaries between common and selfish interest shift continually in fact, depending on circumstances, and also in our estimation as information alters in quantity or weight. Thus, the compromise fixed in our political systems and in our laws is always obsolete, and only approximate at best. Discontent is inevitable.
2. In order to serve the common interest it is necessary to devolve and depute some power to a subset of the population, the police, the government in general, loosely the civil and military services of the state, or The Crown. But these people are self-interested people like any other, and will inevitably and even if only fractionally and subtly abuse their power, overstating the degree of the sacrifice that the general population must make to secure the common interest. Equally, the rest of the population will, individuall, underestimate the sacrifice and seek to benefit without contribution.
This tension is inevitable, but note that the civil and military services are co-ordinated and well-armed, by virtue of the legitimate tasks that they must perform, and have indeed a common-interest in exploiting the mass of individuals, whose those individuals themselves are poorly armed or co-ordinated, except at time of rebellion, and have only a weak common interest, partly because of the cost of co-ordination, which the civil and military powers have as a free gift of their work, and partly individuals in rebellion are very unevenly exposed to hazard in revealing and prosecuting resistance.
From these points flows the ebb and flow of political history, with individuals ever dissatisfied with the extent and character of their government, whether in the person of kings, civil servants, soldiers, politicians, and clergy. It really is that simple.