Why would Nick Clegg say that? Why is Nick Clegg so pro-European?

Nick Clegg has warned David Cameron that Britain must not back out of Europe and dismissed plans for a referendum on the country’s membership of the EU as “putting the cart before the horse”. Why would he say that? Why is Nick Clegg so pro-European? Is it anything to do with safeguarding his nice EU pension? Remember that EU pensions are paid on the understanding that the EU can remove this pension, if in the view of the Commission or the Luxembourg Court, they "fail to uphold the interests of the European Communities".