I was reading the wikipedia article on Macedonia naming dispute. As a reasonable human being, I can't believe that just by debating how a territory should be called, these people hindered economic and trade relations, potentially reducing the living standard for their populations (20 million people). Still Greece is blocking Macedonia's EU accession. Why is it so important to name something so that millions of people have to suffer the negative consequences? It kind of reminds me of kindergarten when we were arguing about what we should call our superheros. But at the international level? What can't they just simply come up with a new name?
As a reasonable human being, you should also probably be fully aware that:
Splitting off territories based on nationalist movements is usually Bad JuJu for anyone involved, for a variety of reasons (even if done peacefully, there's negative economic impact in the beginning, and there are concerns with well-being of ethnic groups native to the former large state who are now a minority in the new state - e.g. see Russians in Baltic exUSSR republics).
Given the territory in question (Hint: this is where the word "Balkanization" comes from), Greece has VERY valid concerns with possibility that Greek side of Macedonia would want independence and/or merge with FYROM/RoM.
Issues like national identity and such are a pretty big part of human psyche. Humans have been known to kill each other over making ugly faces, never mind something as complicated and influential as national identity mired in 100s (or in case of Macedonia, 1000s) of years of history, conflict and war (anyone remember Philip and Alexander, two minor Macedonian kings?)
Please have a look at Greek Ministry of Foreign Affairs document discussing the issue in diplomatic terms for their officially stated reasoning.
What can't they just simply come up with a new name?
Well that's hard. People are attached to their name, the name of their family, the name of their tribe, name of their city, name of their country. "come up with a new name" means they should change their name based on what someone else wants. Lots of people don't like this, and think that they alone should be allowed to choose their name.