The budget is blowing out for three reasons: Social Security, health care and the military. Of the three only the last is largely discretionary (being the ongoing cost for campaigns in Iraq and Afghanistan).
When FDR introduced Social Security there was something like 50 working people for every retired person and the average life expectancy was probably only 60 or so. It's a straight money goes in money goes out system with no saving or planning for future spending. Everyone knows this is unsustainable. It's merely a question of when something will change and what. The smart money is on raising the retirement age.
Health care costs are spiraling. This is a much more complicated and multi-faceted problem. Personally I think part of the problem is the blank chewue mentality we have to health care. Basically it doesn't matter how much it costs to research and develop pharmaceuticals and medicine, we expect to be provided it. So companies largely ignore the costs.
It's a bit like saying to a subcontractor: you can choose to spend as much as you like and I'll pay you a 20% margin on top of that. What is the motivation of that subcontractor and the inevitable outcome?
Also I think you need to look at the structure of the health care system. I suspect the US has the most expensive health care in the developed world. I think systemic inefficiencies and overprotective IP laws play a bigpart in this.