A "Currency Area" has benefits and drawbacks. The benefits are mostly microeconomic in nature. For instance, if I can contract to buy something from you in the future without having to hedge currency risk or change money, that's a benefit.
The drawbacks are mostly macroeconomic in nature. For instance, Germany is doing OK now while Greece is suffering. However, they share one currency (the euro). If they had two currencies, Greece could devalue its currency, which would boost its economy.
The drawbacks can be mitigated by things like labor mobility, e.g. Greeks fleeing Greece to go to Germany where the economy is better. This happens in the US all the time. However, Greeks and Germans don't share a language, so this limits labor mobility.
Similarly, price/wage flexibility is important, but again, Europe does poorly on this test because its unions and work rules.
Another way to mitigate currency area drawbacks is for the region to have a "transfer union" that can use fiscal policy to spend more money in struggling regions. However, Germans don't like the idea of giving money to profligate Greeks who waste it (this is their view).
All in all, most economists don't think that Europe is an Optimal Currency Area and that weak countries will be kicked out of the Eurozone.