Here is a graphic from NPR that shows the ebbs and flows of US Federal Budget SPENDING as a percent of Gross Domestic Product.
I doctored it a bit to show the change in the composition of Federal spending in the categories of Non-Mandatory ("Discretionary") and Mandatory (Non-Discretionary") from the 1970's to 2013.
We used to be able to "do things" with Federal dollars. Now we transfer Federal dollars from one person to the other. If the budget is relatively fixed (in terms of a percentage of the economy) and you are forced (by demographics) to re-allocate that fixed budget in different ways then something has to give...Right??
Just throwin' it out there...