There's a difference between being in the EU and being in the Euro.
The EU started as a trading community aiming to reduce customs barriers and encourage free movement of goods etc.
Later ideals of common laws, tax bases, currencies, armies were mooted.
While the idea of having a common currency encourages tradeflows, the difference between economies - what they produce, how people are taxed, what the state chooses to spend taxes, and how much they choose to/have to borrow - causes conflict. Only if a federal model for Europe were adopted would a currency have a chance of working well in the long term (which is why the USA works despite being larger and having widely differing local economy) And lots of people do not want a federal Europe because, as was stated above, there are differences in laws (and even the concept behind legal systems) customs and political systems that would have to be accepted; you can't make all the people happy all of the time