Dude,that's like asking why World War II started.

HOW WWI started:

On 28 June 1914, Gavrilo Princip, a Bosnian Serb student, shot and killed Archduke Franz Ferdinand, heir to the Austro-Hungarian throne in Sarajevo. Princip was a member of Young Bosnia, a group whose aims included the unification of the South Slavs and independence from Austria-Hungary. The assassination in Sarajevo set into motion a series of fast-moving events that eventually escalated into full-scale war. Austria-Hungary demanded action by Serbia to punish those responsible and, when Austria-Hungary deemed Serbia had not complied, declared war. Major European powers were at war within weeks because of overlapping agreements for collective defense and the complex nature of international alliances.