If Greece were to withdraw, the government would find it unnecessary to pay its outstanding euro bonds. German, French, and Swiss banks are the largest holders of Greek bonds. They would not be pleased if Greece defaulted on the bonds.
From this article:
It remains unclear whether it would even be legally possible for Greece to depart from the euro zone. Legal experts believe it would also be necessary for the country to split from the European Union entirely in order to abandon the common currency. At the same time, it is questionable whether other members of the currency union would actually refuse to accept a unilateral exit from the euro zone by the government in Athens.It looks like it may not be up to the Greek government on whether or not it can leave the euro zone. If not, then it is apparent that Greece is not a sovereign country.
This reminds me of a historical episode where some states attempted to secede from a union. That didn't go well. I hope the Greeks have better luck.