The biggest contributor to why the US keeps bumping its head on the debt ceiling is INTEREST on the public debt.

Even if Congress were willing (or gutsy enough) to refrain from spending more than comes in, we'd still have a recurrent debt ceiling problem (absent a stringent tax rise), because of the compound interest on the debt which Congress has built up over many decades, and the interest on it, and the interest on the interest load.

Dave Oesterreich