One of the main reasons Serbia is against the indepenence of Kosovo is that it regards the battle at "Kosovo Fields" as a defining moment in the beginning of their nation even though the Serbs were defeated.
(http://en.wikipedia.org/wiki/Battle_of_Kosovo)
A small quote:
"The Battle of Kosovo is regarded to this day as a milestone in the Serbian national identity. It remained a symbol of Serbian patriotism and desire for independence for centuries under Ottoman rule. The Battle of Kosovo has had recent impact in the Kosovo War cultural relations remain tense as the majority Muslim Albanians claim that their majority should grant them the right to independence while the minority Serbs claim that, partly due to the famous battle, Kosovo is still an important and integral part of Serbia."
The speech given by Milosevic at Kosovo Fields in 1989 indictated that Serbia under no circumstances would relinquish their control over this area.
Another main reason is that the Serbs reguard themselves as doing Europe a favour by stemming the flow of Muslims into greater Europe.
Part of his speech on 28th June 1989:
"Six centuries ago, Serbia heroically defended itself in the field of Kosovo, but it also defended Europe. Serbia was at that time the bastion that defended the European culture, religion, and European society in general."
At the time a friend of mine who was born in Australia of Yugoslavian heritage said to me, "Serbia will never ever let Kosovo become independent as it is too important to Serbian history" I must point out that whilst her mother is ethnically Serbian, her father is ethnically a Bosnian Muslim, so she can't be seen as having any bias.