Serbia’s sense of nationalism added to tensions in Europe before WWI because it set stronger countries against one another. It also helped lead to the assassination of Archduke Franz Ferdinand, which, when combined with Austria’s ultimatum, caused WWI to begin.
Serbia was, of course, a country made up of Slavic people. This meant that Serbia was related, in a sense, to Russia, which was also a Slavic country. It also meant that Serbians were related to the many Slavic people who lived in the Austro-Hungarian Empire. Austria-Hungary was ruled by ethnic Germans, but it was made up of people of many different nationalities.
Serbia supported Slavic nationalists in the Austro-Hungarian Empire. This caused tension between it and Austria-Hungary. This also led to greater tension between Austria-Hungary and Russia since Russia was allied with Serbia. In turn, this led to tensions between Russia and Germany because Germany was allied with Austria-Hungary.
Serbian nationalism eventually led to the assassination of Archduke Franz Ferdinand, who was an ethnic German ruler of Austria-Hungary. When Serbian nationalists assassinated the Archduke, Austria-Hungary used that as a reason to push Serbia into war. Thus, Serbian nationalism led to the assassination, which increased tensions dramatically.
We can see, then, that Serbian nationalism fueled tensions in Europe over a long period of time. Eventually, it led to the focusing event that caused the beginning of WWI.