As I understand, becoming inseparable friends is vastly helped along when there's someone else to hate instead, suggesting there's always got to be a "bad guy".

In my course on the EU the instructor asked why the American colonies formed into a cohesive nation while the EU is still largely fragmented. Of course, there are a lot of answers to that, but I pointed out the colonies had to unite to stand against Britain. The EU hasn't had any external entity to unite against, thus the focus remains on the differences between the member states rather than the goals they share.