Three, interrelated reasons, mostly having to do with the strain on the middle class.
First, we have no manufacturing base to stabilize the middle class. Our economy is increasingly monopolized by the serivce industry. We don't make anything. We can debate why that is -- broader education means people do not have to be in labor as much anymore, lower cost of production in other nations, etc. Bottom line: the country does not represent the starting point for production of goods to nearly the same degree it used to, and that leaves many more people without any economic power.
Second, the upper class is pulling away from the middle class and that is destabilizing the political framework of the country. We have always had a super-wealthy class. But 250 years ago they did not live that much better than the blacksmith. And wealthy industrialists at the turn of the 20th century did not have nearly the same magnitude of wealth that the current Wall Street and tech investors have relative to the middle class now.
This country is producing exponentially more millionaires and even billionaires than ever before, but the middle class remains firmly stuck in living paycheck to paycheck. I do not bring that up as class warfare -- it is simply a fact that a broad and stable and satisfied middle class is the key to moderate politics. But the greater the distance between the upper and middle classes in this country, the less willing the middle class is to foot the investment returns for the upper class with their own labor. (This is not to say that its hard to become upper class -- it is easier now than ever before. But it is to say that if one struggles and cannot get there, then there is an increasing resentment. The election of Obama is driven largely by the sense of the minority population that the game has been rigged for so long. Does not help that corporate execs are taking hundred million dollar bonuses at the same time that the middle class is being called upon to pay for a bailout of the exec's company).
Third, all of the economic stresses have led to a real assault on the family. Twenty years ago, the debate was whether a family of 4 or 5 could get by with the Mom staying at home. Now, the debate is whether they can get by if Mom and Dad just work one job, each. The further down the economic ladder you go, the more likely you are to see a dysfunctional or even simply nonexistent family support system. And for the kids growing up in that situation, there is an even greater likelihood that they will break away from the solid middle class foundation I speak of above, and resentment and lethargy grow with it.
Government can have only a very limited role in dealing with these issues. In a macro sense, this is really a growing pang as we shift from industry to service. But there is a very real danger of the country not being able to sustain the image we like to have of ourselves as the land of opportunity while this tranformation goes on. We'll either self-correct it, or a thousand years from now be a footnote with the great but failed empires of the past.