Sovereignty
*At any time*, for no reason, the UK can give two year's notice to leave the EU. They literally can't stop us.
If we stay and the EU ever threatens our sovereignty (even though we are exempt from closer union and can't transfer powers without a UK referendum) we have enough sovereignty to leave.
So why jump now? Don't we trust ourselves to exercise our own sovereignty wisely in future? Or is our Government a bit too sovereign for our own liking?