The main reason lies in the field of economics. The rich North refuses to support the poor South. That is the problem. They don't mind European unity as long as they keep their money. Personally I don't see any future for EU in its present state. It's a confederation and confederations simply do not work. They either dissolve or turn into federations. In order to function EU needs centralized government, banking system, laws, etc. You cannot be unified 'just a bit' and keep the sovereignity at the same time.