Here's a few reasons:
France: principle beneficiaries of the Common Agricultural Policy (one of its its main industries)
Spain: principle beneficiaries of the Common Fishing Policy (big business in Spain)
Germany: Strong economy which wants to flog all its goods and services to the other nations (usually with money borrowed in euros)
Estonia, Latvia, Lithuania, Luxembourg, Poland, Cyprus, Malta, Slovenia, Slovakia: enjoy having a joint bank account (the euro) with Germany.
Greece: wants to remain in the euro and the EU because it affords them a standard of living beyond their wildest dreams.
UK: Fishing industry all but destroyed (see "Spain" above). Farming industry under severe stress because of huge subsidies given to foreign farmers (see "France" above). Finance industry (City of London accounts for almost 40% of all finance transations in the EU) under constant pressure from the EU which seems intent on destrying the City's dominance. All financial regulations are now voted on under "Single Market" rules (even though there is not and almost certainly will never be a single financial market in the EU). This means qualified majority voting and the UK (with just 10% of the voting rights) will be forever outvoted on these issues.
I could go on, but someone will simply denounce my ramblings as a load of old cobblers.