The main reasons why we should leave are:
That we don't need to be ruled by a foreign power to trade in Europe, the EU is undemocratic and this matters. In a multiparty democracy, elected politicians have to face the ballot box, this stops them straying too far from a middle path and doing the best for the overall wealth and health of their citizens. Many of the EU economically damaging policies would not see the light of day if the European commission and their civil servants (the only people allowed to introduce legislation into the European Parliament) had to face the electorate through the ballot box. Likewise, they would not be stuffed with failed politicians like Neil Kinnock, Peter Mandelson etc, or useless jobsworths like Lady Ashton.
A country's sovereignty is like virginity, it is something you either have or you don't. If you do and use it wisely to look after your citizens they benefit. If you don't, you are relying on other nations, with their own priorities to look after you (they won't).
Name one rich country that is not a multiparty democracy based on capitalism. The simple answer is that there isn't one. The UK took a world lead from the 16th century, with a Royal family dictatorship turning into parliamentary democracy and we haven't looked back since.
Europe is getting poorer as a result of the EU and if we allow it to control us until it collapses, we will be a very poor country by then.