Answer:
Imperialism - Nations competed for more land, colonies and raw materials. Great Britain and Germany competed industrially, which led to these nations needing more raw materials. Also, Austria-Hungary controlling the Slavic land of Bosnia, which Serbia believed belonged to them.
Nationalism - Nationalism was very strong in both Germany and France. Germany controlled the territory known as Alsace-Lorraine, which they gained from France after the Franco-Prussian War. France wanted that land back.
Alliances - Secret alliances were formed prior to the outbreak of the war. After the Assassination of Franz Ferdinand the Archduke of Austria-Hungary the alliance systems went into effect leading to many nations protecting one another (i.e. Russia protecting Serbia from Austria-Hungary).