World War I kicked off in 1914 when Archduke Ferdinand was killed in Sarajevo. However, this single action was not enough to cause such a massive conflict. Unlike World War II where the Axis of Evil (Germany, Japan, and Italy) were obvious aggressors, no one evil bad guy is apparent in World War I. Germany and Austria are regarded as the central culprits, but their role is not as concrete.
Political reasons revolved around the mutual defense agreements which had been created over the previous decades. Britain, France, Japan and Belgium were bound through these mutual agreements. Germany and Austro-Hungary also had mutual agreements. Imperialism is another reason countries were pulled into war.
Imperialism is expansion of world power by colonialization. Africa and parts of Asia were still viewed as valuable property and the easiest international political excuse to taking it was through war conquer. The valuable economic boost a colonialized or seized assets of an area could provide a major influx of wealth into a counties coffers.
Societal reasons include the expansion of freedom along with concerns over the economy. Morocco was given to France by Britain, but the Moroccans wanted their independence. Germany sided with them, but eventually backed down when the UK supported France and Germany settled for part of the French Congo. This is an example of the type of large property exchanges going on around the world. People were feeling powerful, industry was increasing and there was a general sense of excitement to explore due to the technology available.