It's the rising cost of health care.
Ezra Klein: What these three charts tell you is simple: It’s all about health care. Spending on Social Security is expected to rise, but not particularly quickly. Spending on everything else is actually falling. It’s health care that contains most all of our future deficit problems. And the situation is even worse than it looks on this graph: Private health spending is racing upwards even faster than public health spending, so the problem the federal government is showing in its budget projections is mirrored on the budgets of every family and business that purchases health insurance.