People do seem to like a few years of "stability" in leadership and them drift to wanting a change as problems and scandals eventually accumulate. Governments also age badly when they become arrogant and dismissive of legitimate concerns about fairness and due process. Even a mediocre opposition can look ready for government if the incumbents start to smell of corruption and display disrespect for voters. Changing leaders can actually help a government appear "refreshed", especially if a leader has become a lightening rod for disaffected voters.
I also wonder whether economic illiteracy (deliberately fostered by politicians and partisan commentators) is also a factor in people blaming governments for their personal circumstances or economic woes generally. We all know that the economic circumstances we experience at a particular time are the result of decades of policy and prevailing and historic international conditions - with the policies of the current government playing a fairly small role. We do seem to still take delight in blaming the current (or immediately former) governments for all our problems if they are not our preferred brand. Conversely, we (and they) claim credit for any glimmer of sunlight.
