Here's a lead-in paragraph to a Brookings Institution article by Douglas Elliott that is a good summation as to why we need to be concerned about the Eurozone economy and its effect on the US economy.
"The Euro Crisis has struck again, hammering not just European markets, but doing real damage to U.S. markets and to economic prospects around the world. The U.S. could easily be pushed into another recession if the eurozone collapsed. We export well over $300 billion a year to those 17 countries; virtually all of the rest of our exports go to nations that also export to the eurozone and would feel ripple effects; roughly two-fifths of our overseas assets are invested in the eurozone; and our major financial institutions have large credit exposures to eurozone banks and other businesses."
source: http://www.brookings.edu/papers/2011/0822_euro_crisis_elliott.aspx#note1, Why Can’t Europe Get it Right the First Time… or the Second… or the Third?