Over a longer term, spending is projected to rise further due to projections of rising health care costs and a growing interest burden, which is the result of a growing debt. The deficit fear mongers like to hype these projections of large deficits decades in the future to advance their agenda of cutting Social Security and Medicare.
The reality is that the story of exploding interest burdens is utter nonsense since there is zero precedent for the country ever allowing the debt to expand in this way. This makes as much sense as arguing that someone driving west in New Jersey risks falling into the Pacific Ocean. People driving west in New Jersey invariably turn or park their calls before ending up in the Pacific Ocean and the United States has always taken measures to reduce deficits long before they posed a fundamental threat to the economy.
The real question is why the primary (ie non-interest) deficit rises and this is the story of the broken US healthcare system. We pay twice as much per person for our health care as the average for other rich countries, with nothing to show for this money in terms of outcomes. We pay 2.5 times as much as the UK. If our costs were at all in line with those in other wealthy countries, we would be looking at explosive budget surpluses running into the trillions of dollars annually.