There are just too many differences, local and regional interests, petty quarrels, abundance of languages, and centuries of resentments separating the diverse peoples of the "old continent"...
It can work at some levels, eg. currency, economics, etc. - due to common markets and the globalization the Portuguese started (read on); but beyond these common points of international intersection, it will be difficult to implement union. The largest conflicts the world has seen have arised from several attempts at the unification of
Arguably the most important factor for European unification, has to be an "European Identity". Europeans need to feel Europeans, first and foremost, and be proud of it. Like, I guess, most Americans feel American through and through, and only then New Yorkers, or Hawaiians, etc. - with possibly the exception of Texans...
How many Europeans love
One of the most European countries of them all, I dare say, is one of the oldest as well: Portugal - and I will readily explain why, after a little introduction.
In the age of Discoveries (from 15th century on), the Portuguese, already has an individual nation, have followed the example of Alexander, being the first country to eagerly mingle with people of all continents and considering them "Europeans", creating a mixed nobility in India and incorporating the "native" or mixed Africans, Asians and Amero-brazilians in religious orders, the military, etc., as citizens of own right, of which there are many examples in history.
While medieval super-powers like Venice were selling out to the Otoman Empire which controlled the spice commerce and used its exorbitant revenue to fund their conquest of Eastern Europe, the Portuguese are owed kudos for having open a new route to the spices and fighting off the Muslim hold of commerce between Asia and Europe – thus much contributing to halt the expansion of the Otomans to the west and initiating the current western “supremacy”. And what could be more European than "crusades"?
Unlike most European States that jumped in the world navigation wagon, lead by the Portuguese, we were driven not merely by economical profit, but also to spread the faith and civilization - which we made the point of sharing with non-europeans, sometimes against their liking … but many accepted the gift of Europesation gladly. And we were also happy to meet them, and their women, being the Portuguese the first Europeans in
So, at the end of the day, you gotta love