So yeah, we always pay our debt. Problem is we are always borrowing more than we have before. We do this by issuing bonds, which are held by both foreign powers (who buy our treasuries sobthatbwe import their products), domestically in private accounts, and intergovernmentally to things like the social security fund. If we actually started defaulting on bonds that were due we would have a serious problem because nobody would then be willing to buy our bonds anymore. We wouldnt be able to function as a government at that point as things stand now.
Also, as debt claims higher in relation to GDP it can cause debtors to demand high interest payments because of increased risk. It also can cause a lower dollar, which in turn makes our bonds less attractive since the payment is worth less relatively. There is also the looming issue of social security insolvency if we cant pay that fund back quickly enough to cover retirements.