Just to be clear, i don't think for one second that leaving the EU is a painless process. However, someone likened it to a divorce and i think that's a good analogy.
Getting divorced is painful, expensive and in the short term everyone involved is financially worse off. It only takes one side to be unhappy for the breakup to take place. It makes sense to try and remain good relations with the other party in the divorce. But in the medium and long term it makes no sense for two parties to remain heavily interlinked purely for financial reasons if they are incompatible in the medium and long term.
There is undoubtedly going to be some short term pain, but in the medium to longer term i think the EU is either going to implode or go down the route of ever closer union to become the united states or Europe, and we will be better off out financially, emotionally and diplomatically.