Why Serbia?

"One day the great European War will come out of some damned foolish thing in the Balkans "

-Otto von Bismarck 1st chancellor of Germany, 1888

The conflict between the Kingdom of Serbia and the Austro-Hungarian Empire was the spark that ignited the First World War. The assassination of Archduke Franz Ferdinand was the tipping point but it was not the source of this conflict. Both nations had good reasons to be fearful of each other. Austria-Hungary had been expanded into the Balkans in mid-nineteenth century (by 1914 it controlled much of what is modern day Slovenia, Croatia and Bosnia-Herzegovina). This region was vital to Austria-Hungary as it is where the Empire's only seaports were located along the Adriatic sea. At this time railroads and steamships were the foundation of any wealthy nation's economy. Free trade was more or less none existent. If  Austria-Hungary did not control a port with ocean access it would have to pay tariffs on everything the country imported and exported that crossed another nations borders.

Serbia was growing in power in the Balkans having achieve much in the First and Second Balkan War (fought in 1912 to 1913). Serbia has aspirations of achieving independence of Slavic regions controlled by non-Slavic nations (and absorbing these regions when possible after independence was achieved). Austria-Hungary controlled many Slavic regions neighboring Serbia that were not enthusiastic about being under Austria-Hungarian rule. The Kingdom of Serbia had no hope of facing Austria-Hungary in a direct war to liberate these regions. Austria-Hungary was larger, more wealthy, and had a much larger population and army than Serbia.

Even though Serbia knew is could not face Austria-Hungary in a war, it could (and did) offer support and encouragement to Slavic political groups (some peaceful and some not) in Austria-Hungary that wanted increased autonomy (or outright independence) from Austria-Hungary.  

The Slavic regions of Austro-Hungarian Empire that the Serbia supported Slavic independence political groups (and arguably some groups we could call terrorist today) were the same regions that Austria-Hungary relied on for its national seaports. Austria-Hungary could not afford for these regions to be destabilized (or allow them independence). Economically Serbia's actions were very threatening to Austria-Hungary.

At the same time Austria-Hungary was an Empire with aspirations of expanding its power in the Balkans (as Russia's and the Ottoman Empire's influence in the region was waning and the Austro-Hungarian Empire was eager to fill the void). So in the mind of Austro-Hungarian politicians a war with Serbia would kill two birds with one stone. If would put a stop to Serbian meddling in its vital Adriatic coastal region and expand its power in the Balkans as Russia and the Ottoman Empires withdraw interest from the region.