It's like going to the restaurant and not only having to pay for your meal but the older folks at the next table too. Why would they vote to pay later for our spending now. I think it is because they don't understand what is happening.
The details are not partisan. This is not a Republican or Democrat problem, it is a math problem. Those in the know have recognized this problem for years. If you want to become more literate on this subject I recommend any of these books:
The Coming Generational Storm, written in 2003, shows that this problem was not something we were unaware of. The author updated his material in 2012 with The Clash of Generations
Comeback America, written in 2010, is written by the former CBO chief and is a very readable book on our federal deficit problem.
Red Ink, written 2012, is shorter than the other books and even easier to read.
In a perfect world, you would be required to read one of these books before voting.
For those with shorter attention spans, start them off with one of these Youtube videos: