There is no simple answer. The current situation, however, is largely the result of the 2008 financial crisis. And this was the result of lousy government regulation, financial institutions that took ridiculous risks for short-term gain, and many Americans living beyond their means (using the inflated prices on their homes as a source of credit). The bubble took years to form, and it will take years to unravel. Meanwhile, European countries with overgrown public sectors saw tax revenues fall with the global economic crisis, and now the day of reckoning has arrived.
Some, of course, will claim that it is all the fault of either Democrats or Republicans, Obama (or Clinton) or Bush, but anyone who buys into these ideas has not looked at the situation very closely. All sorts of people in the private and public sectors are liable, and the smart ones were those who walked away with tons of money before the bubble popped.