This is really very simple.
The Euro has strict monetary policy. This favours economies like those of Germany, and makes it very difficult for economies that often need to dilute their currencies to stay competitive.
In a 'currency union', basically, there can be no shenanigans. It has to be a 'strict' currency, because it's not like the French are going to allow the Poles to just start 'printing Euros'. So, by the very nature of the shared currency, it's going to be strict.
When Italy, Spain and Greece start to have trouble making bond payments - this poses a problem. They can't just start printing Liras or Drachmas anymore.
When their economies are faltering - they would normally print money, dilute their currency, meaning everyone in the country is taking a secret pay cut, retirees get less benefits vis-a-vis other countries. But it's stealthy. Nobody sees it. But in Euros ... everything would have to be repriced for the economy to remain competitive. People have to accept lower pay. Retirees lower benefits. That is politically impossible.
And so you end up with high unemployment for a very long time while it sinks in that they are poor.
Germany was very wary of entering into the Euro because of their historical fears of currency devaluation. It was a big thing for them, to the point that it was a major factor in the rise of Nazism, and that whole WW2 thing. So when they did it, there had to be controls.
Most citizens of Europe would prefer a more trade-like situation, not a true political union. The EU is ultimately an elitist project and it's showing serious signs of strain.