Come again?
Gladly.
At the present moment the US national debt is extremely cheap to service due to low interest rates. When interest rates get around to rising, servicing the national debt will become impossible.
There are only 3 ways to get rid of the debt. Growth, inflation, default. Growing our way out of it is not going to happen; look at GDP numbers and budget deficits. Inflation is a possible, but that much inflation would turn the US economy into Zimbabwe or the Weimar republic. So defaulting actually starts to look attractive.