Another thought-provoking post from Bronte: (emphasis mine)

I am going to do a little further explaining of the stakes here.  The threat to currency union in Europe always was ultimately racism. 

When you have currency union you can no longer have a high interest rate in Spain or Italy (when those economies warrant a high rate) and have a low rate in Germany when Germany is recessed.  You have a single interest rate across the currency zone. 

The underlying state of most of the past 15 years was Spain booming, Germany mildly recessed.  Currency shifts or shifts in value between the Deutsche Mark and the Peseta can – by dint of currency union – no longer happen.   The main mechanism of economic adjustment is removed.

America has always done that.  The same interest rate applies in the rust belt, in the sun belt, in California and in Boston.  And we know how economic adjustment happens.  Americans move.  A vast number of Americans do not live in their home town and the bulk of the world’s busiest airports are American.  Internal American migration is massive.

However migration within Europe has always involved more issues.  The languages are different.  Several countries have histories of nasty endemic racism.  There are large cultural barriers.

Labor migration is key. Not only geographically, but also the ability to reduce the friction in adjusting your labor force's skillsets. For example, as America shifts more away from manufacturing into services, it's critical that we support our past workforce - this is where I (philosophically) break from more extreme Libertarians - the government has a role in reducing that friction across generations (and sometimes, you have to protect your labor force - although it should always be with an eye to the future).