I am referring to the past when there was less divisive leadership. I think the good things EU offers are generally a great idea - ease of transport, trade, common markets, common currency (* - not so certain on this one, but I'm tentatively positive), and so on.

But when you do things like force countries to accept immigrants they may not want to accept, or demand cultural conformity on issues that are not well received, you're undermining your union. Historically religion is a great example here, as it was always the thing that divided people the most. And the Empires that thrived were those that allowed people to maintain and practice their own religion. In modern times socio-political values largely act in the same role, and also form the same divisions. The only difference is it just comes from personal ideology instead of scripture. Unfortunately we still seem largely stuck at the era of calling each other blasphemers and calling for crusades.