Debt has legitimate uses. Why shouldn't the state use the money of foreign creditors to prop up its economy? That benefits your economy at the cost of everyone else. The problem is when it accumulates to the point it cannot be paid back. More specifically, as the state's debt load increases its credit worthiness declines. At some critical juncture--which is impossible to predict in advance--you get a sovereign debt crisis in which your creditors stop lending and your currency becomes worthless as you run the printing presses to inflate your way out. The reason that point can't be predicted is because it's an effect of herd behavior in markets. You can never know in advance just when investors will get spooked.
All the debt doesn't have to be paid back. It simply has to be kept manageable. If it grows at a rate below that of GDP + inflation, it's not a problem.
There's been some talk of "balanced budget" amendments to the US Constitution. I'd really rather see the government begin to take a long term view on things and get the growth of deficits under control...because that makes it harder to use debt to prime the economy, but it's hard to imagine Congress getting their house in order.