Why were American economists jealous about the Euro?

Two economists in the European Commision published a nice paper on "The euro: It can’t happen, It’s a bad idea, It won’t last. US economists on the EMU, 1989-2002" arguing that:

"The study of approximately 170 publications shows (a) that US academic economists concentrated on the question "Is the EMU a good or bad thing?", usually adopting the paradigm of optimum currency areas as their main analytical vehicle, (b) that they displayed considerable scepticism towards the single currency, (c) that economists within the Federal Reserve System had a less analytical and a more pragmatic approach to the single currency than US academic economists, and (e) that US economists adjusted their views and analytical approach as European monetary unification progressed. In particular, the traditional optimum currency approach was gradually put into question"