Exhausted Ever closer union; the answer to every problem being 'more Europe'. The 'voice' we had being diminished by QMV, and more and more changes being made as they are decided as non legislative, as opposed to them going to the Council or to national parliaments. Being subject to the ECJ, and the ratchet of the Acquis.
I think that some don't look closely enough at what the EU is up to, seeing it as a fluffy unicorn glitter cuddly organisation that makes nice rules about mobile phone chargers. It isn't.
The rest of the world seems to manage, and prosper in many cases outwith the EU. Why shouldn't we?
Brexshitisntit see above. Add into this being on the hook, even though we are outside the Euro, when the next global financial crisis hits, as it may well do shortly, to drag the poorer EU member states out of the mire. I also dislike the way the rules are ignored - Germany is not fined for the budget surplus it runs outside the parameters growth and stability pact - France and Belgium aren't fined for breaching those rules either by having too great a deficit. The rules aren't applied evenly, but to exert political pressure, or to buy political capital with particular member states. I think that's negative.
Inion I don't trust the EU as the processes are so opaque; the language can mean whatever the EU wants it to mean on any given day; rules are broken, without consequences, by member states and by officials; and it is all about the Commission ultimately possessing central control via the Acquis.
Cross national cooperation seems to work fine in NATO without ceding control of our finances, foreign policy and submitting to the laws of an external judicial system, and our parliament subservient to a supra national organisation.