To most people, the personal may not be the political, but the political is personal. Average people support a leader based on their knowledge of his history, and his personality. They do not support leaders based on rational thought about, or understanding of, policy. Charisma is obvious enough, even if hard to explain. Some men are simply likable, either in person, or on TV, or -- hopefully -- both. This gives them an advantage in politics, as in other things. But many men have charisma; it is not enough to explain politics.
What I want to look at here is the effect of participating in certain rare communal happenings upon politics. This seems to work as follows: a large social movement happens; a common understanding of that movement is formed which emphasizes its goodness; and from that, the men who led and/or even participated in the movement (on the post-facto winning side), are deemed good leaders. They then proceed to control politics.
There is one main form of a social movement: a full-scale war. Other events that seem to generate the leaderly aura are revolutions (though this usually involves at least some war), and mass resistance movements to unpopular governments.
How can we tell apart "full-scale" events from lesser ones? In America, it's pretty easy - look at the presidents. The full-scale events generate two generations of presidents; a first generation of the leaders of the movement, then a second generation of the "soldiers". Consider the (first, successful) Revolution. Every president from Washington through the second Adams was involved in the Revolution. That's a period of 40 years; but that does not count the 6 years under the Articles of Confederation, and the period with no formal government at all during the fight.
Or consider the Civil War. (Really, "Civil War" - scare quotes are appropriate when calling a failed war of secession a civil war.) Afterwards, every President except one until 1901 served the Union in the military. (The one who didn't, Cleveland, drew straws with his two brothers for who would pay a replacement and stay home; he lost.) That's a period of 35 years.
Or the final big event: WWII. All of the presidents from Eisenhower until Clinton, excepting one (Carter), served in the war. (Carter was a naval officer commissioned just after the war with honorable service; so his service was not a political issue. Reagan is an exception proving the rule: he "served" as it were from Hollywood.) That's 38 years. George W. Bush would not be president but for his name, and thus, there is a lingering effect of WWII even now. (That's probably also true for those earlier events; this one I just happen to know more about.)
The obvious deduction to make from this data is that participation in a defining event, like a popular war, will monopolize leadership - until the set of people who participated are literally too old and feeble for politics.
Is this just an American phenomenon? No, I don't think so. I can think of one good example immediately (more in a bit). But it's clearly dependent on the political system. Here's a counter example: I looked at the British Prime Ministers, and there seems to be little pattern there of war service. This may be because getting to be PM is a matter of intra-party popularity more than popularity in the general electorate. The PMs seem to be of the Newt Gengrich mold, not the Clinton one.
But anyway, what's another good example? The USSR. Surely the revolution (and subsequent civil war that really was a civil war) was a defining event. So the regime, even though it was one of the most murderous in history, would have lasted at least until 1917+40 years - 1957 or so. But of course WWII intervened, and the USSR got a new war of national defense to relegitimize a new generation of leaders. And so it lasted until the end of hostilities, 1945, plus ~40 years. Here's a description from the page linked above:
Brezhnev and his buddies, Kosygin, Chernenko and Andropov, were old men who survived in office for just a brief period of time. By the 1980s, political life was suffocating and the political system had ossified. Marxist-Leninist ideology had long since turned into what one Soviet official called "stale bread." The condition of the leadership was a metaphor for a system that was itself dying. Brezhnev died incompetent at age 75 in 1982, Andropov in 1984, and Chernenko barely lasted a year having died in 1985 at the age of 73. The Soviet system was regarded by increasing numbers of people with cynicism, contempt and ridicule.Gorbachev was the first General Secretary to not have served as a political commissar in WWII. He was relatively young, and certainly dynamic. But he was the one that lost control.
So the general theory is: defining event, then 40 years of political control by the men who participated. Toward the end, if the system is not open the leaders will be doddering old men. After that perhaps a few years of drift with a younger leader; he may look and sound good, but he will not have the credential to lead. Then, change. To an open democracy, change is not a problem. But change comes hard to authoritarian states.
Now the fun part: predictions. The theory tells us that all the nasty states founded until about 1960 should have already fallen apart, changed dramatically, or at least become desperately weak. The ones created during the 60s may have a few years left, depending on the flukes of longevity of their leaders.
In particular, several interesting cases present themselves.
China - revolution ended in 1949. Theory says it should have collapsed in 1989 or so. It did not, but the market liberalization has been appeasing people starting about 1980. Prediction: since there is nobody left with revolutionary credentials, the reforms will be impossible to undo, and if the regime tries it will be swept aside. It's possible, though, that the Cultural Revolution counts as a defining event. In which case we should look for substantial change in just a few years.
North Korea - the war ended in the mid fifties. Kim Il Sung used that to rule. His son is riding the tiger; he has no legitimacy, and probably knows it. North Korea will throw him off in the next few years.
Cuba - the revolution was Castro. When he dies, the regime changes. Any year now.
Iran - the revolution was in 1979. We can hear the desires of some of the younger people for change now, but nothing will happen to the regime for another 15 years or so.
I think America should be isolationist - pull back from the world and just trade peacefully. But let's assume the role of a global social engineer, like the Democrats and Republicans. What do we need to do about the "Axis of Evil"? Contain them, at most. They will fall apart, just as other tyrannies have, when the old revolutionaries die off.