the first time European nations have tried a monetary union, it also failed. The only reason why Greece hasn't elected to leave, or that Germany hasn't forced them out, is one of fear.
Greece fears the short term chaos that will result from being forced from a stronger currency, with lower interest rates, into a weaker currency, with much higher interest rates. Their government is already at the point where they can no longer cover their deficit spending through borrowing. The Greek government would like the wealthier nations to help foot the bill with continued bailouts; whose the stipulations attached to them the Greeks have no intention of honoring. However in the long run, either the wealthier nations will become so aggressive trying to enforce their stipulations, or the Greeks their is no saving the system. That they will have to cut government spending; and the only way to make it tolerable is to foster economic growth. And as long as the Greeks are a part of the Euro they cannot grow. They ultimately need a weaker currency, because it will give them a competitive advantage over German and French goods. Reinstating the drachma will allow the Greeks some measure of control over their situation, they have none currently.
The Germans are afraid that the exit of Greece from the Euro could have unravel a system that has profited them. The Germans are a net exporter of capital and the EU has allowed them an avenue to grow by making it cheaper for other European nations to buy their goods. Nations like Greece, were able to borrow at lower rates than they would otherwise have gotten with their own currency. This leads to a growth of both private and government spending, which Germany, and other nations like Germany, were often the beneficiary of. This system has worked very well, until now. Now that these nations cannot sustain their borrowing habits Germany finds itself in the unenviable position of covering the tab. The Euro also benefited Germany as it is the strongest member of the union, allowing to do economically what it could succeeded in doing militarily; and that is control the currents of Europe. If Greece leaves the Euro it could very well threaten this economic system.
By itself Greece is rather insignificant. It's GDP is $ 300 billion, which is less $ 200 billion less than the cities of Paris or London. What the Germans fear is that this could cause a ripple affect amongst the larger, but still economically, shaky nations like Italy or Spain, whose GDPs are each over a trillion dollars. If Greece opts to go its own way then Italy and Spain could decide leaving is an option for them as well. This could result in a currency collapse. A collapse of the Euro would hurt Germany, however, its economy is strong enough that in the long run their economy would be fine. Ultimately the real reason why the political leadership of Germany doesn't want the Euro to collapse is because it doesn't want to risk a fracture of the EU. Neither does the political elite of France, Brussels, Italy, and many other nations.
The elite of Europe truly believe that patriotism, or nationalism as they would derisively call it, is bad for Europe. They see the EU as the best chance to refocus the world back onto Europe, reinstating the world order as it had existed before the fall of the Berlin wall. And perhaps make Europe, and not America, the pivot point on which the world turns, as it had prior to World War II. What the elite wants to do above all else is save the EU. It looks like the European elite are starting to circle the carts.