You have Greeks in the Macedonia region who are themselves refugees from either the Ottoman Empire or the Soviet Union. They moved en masse to the region while at the same time Pomaks and Bulgars and Slav speaking Greeks were moved out (for many reasons, losses from Balkan Wars, siding with the Nazis, or also aiding the Communists for the Greek civil war). Karaskidou's book Fields of Wheat, Hills of Blood does a pretty good job of showing the ethnic mix over the last century in this area. Outside of Greece, Albanians and Macedonians are aggrieved for land losses. Meanwhile, inside of Greece, you have Greeks aggrieved for their own land losses elsewhere. Plus, ww2 and the civil war were not only "cause" for expulsion of other ethnic groups, but a good number of Greeks as well (such as my uncle, a 12 year old drafted into the ranks of the guerillas, he lived in the Eastern Bloc up until the early 1990s). This is why Greek sentiment is so adamant about the Greek part of Macedonia. It's a rather recent addition to Greece, and it was gained through much bloodshed in Balkan Wars, WW2 (against Germany, Bulgaria, Albania, Italy, 1 million Greeks died), and the Civil War.

In Macedonia, meanwhile, you have a national crisis right now because former Macedonian Presidents have become Bulgarian citizens and moved to Bulgaria, as have a great many young people, which further fuels Bulgarian arguments that Macedonians are simply Bulgarians converted to a national mythology by Tito. Ultimately, I think all this mixing speaks to our tenuous identities and the bankruptcy of romantic nationalism. Let them call themselves whatever they wish, whoever they think they are, but maybe to avoid future bloodshed a distinction could be made between Greek Macedonia and whatever. New Macedonia as the US recently proposed?