@Spyman:
There are two questions at work here:
1. Why do we have a debt?
2. Why do we have so MUCH debt?
To answer one, the United States actually began its political life in debt; the Flounders, Hamilton chief among them, felt that a reasonable debt was a good idea to start, as it would 1. Allow the nation to spend from the start to grow and build the new nation 2. A federal debt allows for federal porjects and works, which might unite the nation, and 3. A debt insures a certain amount of attention from abroad--ie, our creditors--and would open the door for relations.
So this is where it comes from, and for a new nation starting up in the 18th Century, at the height of the Industrial and coming into the Imperial Ages, this wasn't a bad idea at all.
So why do we have so much?
Like any good idea, the extreme of that idea can turn out to be a rather poor idea indeed. As the nation grows, so does the debt, because more NEEDS arise...
A small infastructure and little federal intervention is fine for a small, 18th Century state, but not for a 21st Century superpower, THAT requires an enormous amount of spending.
We can't charge the states, directly, as 1. A shared debt is, again, a lareg unifying factor (for example, if we forced California to pay entirely for itself, it has far less of an incentive to state with the rest of the nation, and the same holds true of Texas and Alaska and the test of the States, a shared debt means a shared responsibility.) and 2. To do so would be unfair and tricky, considering the state of the debt currently, deciding who owes what.
Why else do we have so much?
Again, we must have a large military, a large infastructure, update and repair that infastructure, constantly be innovating scientifically and medically to try and stay in line with the rest of the world, as history teaches us that if you fall behind the Scientific Arms Race, you're in trouble, and then you have all manner of public works and services, everything from roads to police to the FBI and CIA and educational services and mandates, social security to provide a safety net for seniors...
And the list just goes on and on.
What do we cut, then?
It'd be easy if the parties could agree, but as America is essentially a nation built on two political ideals, we see the conflict in cost cutting we do today:
Locke/the Left vs. Hobbes/the Right.
Locke believed Liberty preceded Security, as without Liberty, to him, there is little point in being secure, and "comfort" falls inbetween the two.,
So Locke--and the Left--promote ideals such as Education and Caring for the Sick (ie, Health Care) over Defense.
Hobbes lived during the tumult of the English Civil War, so he thought--and the Right agrees--that Liberty is useless if you're constantly udner fear of death or living at the point of someone else's gun, so it's best ot have Security and Defense precede Liberty and all the programs (Education, Health Care, Social Reform) that entails.
The Left points the finger at Military Spending.
The Right points the finger at Health Care.
And neither are totally wrong.
I'm Left-of-Center, so I lean towards cutting Defense first, but I see the validity of both viewpoints.
And THAT is why we have a debt.
No easy, whole-cloth, "it's the Democrats'/Republicans' fault" nonsense.
We were a nation born in debt.
We've lived most our history in debt.
The only way to grow a nation and defend it is by spending.
And so we spend.
And so we grow the deficit.
And so we have one group calling for Defense to be cut over Domestic Services.
And so we have one group calling for Domestic Services to be cut over Defense.
And so we stay in debt, gridlocked in a tough battle of ideals.