Reasons why Greece is acting like this 

One of the main rights of humans, as well as countries is the right to freely choose its name.  One of the biggest insults for me is when somebody is trying to change the name of another one.  This time I will not respond to the insults posted by User:Makedonas, User:Nestore and User:Vergina because the language they are using is unacceptable for a serious site like Wikipedia (Just to quote the last comment by User:Makedonas: This is a country which was a part of Jugoslavia,where a lot of people are Albanians,where they speak a bulgarian dialekt,and they want to have a greek name)!!!???. 
For me it is very difficult to understand the offensive position of Greeks trying to change the name of its neighboring country, so I will try to dig in the history and in the recent events in order to find an answer why Greece is doing this.
 History: Undisputable fact is that modern Greeks have very little common with the ancient Macedonians, even with the ancient Greeks!  Actually, all modern nations living in the Balkan Peninsula are modern nations created with mixing the old nations like ancient Greeks, ancient Macedonians, Ilirs etc with the modern Greeks, Slavic population etc.  For example, the most dominant opinion in our country is that the modern Macedonian nation is created as mixture of ancient Macedonians and Slavic population.  Probably the Modern Greek nation has origin from the ancient Greeks and ancient Macedonians, but that doesn’t give him a right to claim ownership of the name Macedonia.
 Another interesting fact is that the name Macedonia was virtually forbidden for use in Greece before seventies in the last century.  Actually, the northern part of Greece was called “Northern Greece”. Meantime, Greeks realized that they can benefit more if they adopt the name of Macedonia. Starting from that point, the northern part of Greece was renamed to Macedonia and many shops, airports, streets etc got the Macedonian name.  So, Greece is using the name Macedonia for its northern part for 30 years, but Republic of Macedonia is using its name officially from 1945 (that is 60 years)!
 One of the biggest fears of Greece is its Macedonian minority. It is known that a large population of Greece claim Macedonian ethnicity. Actually, during the Civil war in Greece from 1945-1949 a real genocide of the Macedonian population took place there.  Many of the Macedonian people were tortured and expelled from their homes.  Nowadays Greece doesn’t recognize the Macedonian minority and forbids the use of Macedonian language, culture, education etc.
 Very interesting fact is the following: Luxembourg and Belgium are neighboring countries. Belgium has a province which is called LUXEMBOURG! So far, Belgium never disputed that.  In our situation, Greece has a province called Macedonia, but aggressively disputes the right of our country to use its name.
 Greece claims that Macedonia has pretensions over Greek territories. Even if that is true, Greece is several times stronger than Macedonia economically, politically, military etc, so it is really hard to believe that Macedonia can harm Greece, especially by using the name Macedonia.