Why was there tension in the Balkans?

This is something a person that hasn't lived there will never understand, no matter how much you try to explain it to them, but I'll try anyways. There has always been tension in the Balkans, ever since there was the nations which are there today. Nationalism is one of the major reasons for tension. There have been hundreds of wars that take place fully or partially in the Balkans, for various reasons. Every single nation there has a party which wants to take a piece of it's neighbour and make a 'united' country. The people are generally tough with a mentality Western society would call disfunction, have high morale and during times of conflict, extremely brutal.