I'm not sure I'd go with Nationalism per se..
Putting it more broadly, it's a question of identity.
Why are WE forking out for THEM?
Well, if WE regard THEM as more of US, it's more or less OK. We have regional aid, redistribution of income etc. This is starting to cause problems as far as the Union goes. The questions of why WE should fork out for THEM and why THEY shouldn't fork out more for US, are always central questions in politics which get dressed up in various ways.
However, if we regard THEM as OTHERS; foreigners in a foreign country having a free ride, then why are WE forking out to subsidise that lot, people who are in the mess they are in because that's the way they've always carried on and there's no reason they'll change any time soon. Now, if THEY want to carry on like that, that's their business, but it's nothing to do with US. I suggest traditional nationalities are much easier to identify with, against Club Med than a bloc of countries with similar economies but with difficult past relationships and different languages and outlooks, e.g.Holland and Germany.
Now the point of the EU is to get us not to regard the Greeks as foreigners in a foreign country, but all citizens of the EU.