I'm aware everyone in the neighborhood loves mapping their dreams. Does not mean FYROM has any realistic chance against Greece, even discounting NATO membership of the latter.
I'm not saying it is rational to an outsider such as you or me, Yeekim, but this is something that has become an entrenched position.
Think that, at the time of Macedonia's independence from a rapidly-disintegrating Yugoslavia, Bulgaria had just finished expelling hundreds of thousands of Pomaks/Pomaci and there were a crazillion factions disputing control of half the peninsula, with at least three sides, everybody profiting from arming them, and not long after the Albanians in Macedonia themselves rising up against the (until then) peaceful government, borders being redrawn left and right and population being massacred and evicted all the time, all over the place, by every single party involved except possibly the Slovenians, who got out quick. That, plus Tito's old ambitions (again, at the time he had been dead for only a decade) of southward expansion (what would've happened if the central Yugoslav government had won the war? what if the USSR hadn't collapsed right there and then as quickly and bloodlessly as it did?), helped by the memories of (then) recent anti-Greek pogroms in Anatolia and Egypt, and how in the 1970s Greece had been betrayed by all its allies and the invasion of Cyprus tolerated, and you can easily get a paranoid mentality.
Also, you're not looking at it from the POV of the Greeks. To them, ‘Macedonian’ implicitly means Greek. The Slavs are not Greek, do not speak Greek, and try to appropriate Greek cultural signs and identity. They have no problems with Slavic people actually living in (the Former Yugoslav Republic of) Macedonia, but the use of the name sends them over the edge.
This is an emotional issue, not a rational one, but people are far less rational than people usually assume.