Let's pretend Europe is the world (I know that's hard for us Americans who are used to us being the world). Once upon a time every nation in the world had completely separate governments with separate fiscal policies of taxing and spending. They also each had their own banks and their own currency. The Germans had the D. Mark, the French had the Franc, the Greek had the Drachma....
Then one day, some big-brain economist comes down to Earth (again, Europe) and proposes that the world should adopt one currency and therefore one central bank: Euro. Most of the world agreed, and from that day on, those that agreed lost their sovereign control over their own currency but kept control over the taxes and spending. Greece decided to keep spending and spend even more. Meanwhile a recession hits which killed any tax revenues. The result leads to huge deficits that in the past Greeks could handle by, essentially, monetizing the debt - printing new Drachmas and to pay for the shortfall (or put more round-aboutly, having the Greek bank buy up Greek debt instruments in exchange for reserve digits in a computer somewhere).
But this isn't the past. In this new world, there is no Drachma and there is no Greek bank. There is one common Europe bank and the Europe bank, because it's not Greek, doesn't have to play by Greek rules. The European bank, misguidedly, forces strict austerity measures on Greece before providing any money to help out. This results in eventual correction at the huge expense of increased unemployment and a really pissed off (rightfully so) populace.
Now, view the preceding in light our real world (you know, the US). Why should all nations want to give away their sovereign currency and hope that some global bank doesn't' do the same thing to many nations in the next recession the same way the Europe Bank did to Greece?
Greece is probably already screwed, but the rest of us aren't yet. And I happen to think Warren Mosler's idea from a year ago of essentially (and somewhat sneakily) creating a new Greek currency (but not quite) by turning Greek bonds into a form of money (means of payments for public debt - pay your taxes with a bond!) was pure genius.