The ROOT cause, as you imply, was Europtimism.
The PanEurope folks were willing to accept any tissue-paper rationalization or flimsy camouflage to encourage more countries to join in their giant Kum-Bay-Yah fest of the EU.
You CANNOT simply 'bolt' the Drachma and the Lira (sometimes exchanging at 000s to the dollar) to the D-Mark (@2 to the dollar) and assume they're all going to behave like the Dmark. That's ridiculous, if you understand WHY the drachma and the lira were valued so low: a history of unstable government, fiscal irresponsibility, and dangerous levels of debt (considering the former two). As long as states maintained their own spending policies, none of these things were going to 'magically change' once Greeks and Italians now had to sit in meetings with Germans and British. Fiscal prudence doesn't 'rub off'.