I remember studying this back in high school :P
Let's start out with the MAIN factors that contributed to WWI
MAIN = acronym which stands for:
M = Militarism
A = Alliances
I = Imperialism
N = Nationalism
Those are the 4 basic general factors that greatly contributed to the cause of WWI.
For M (Militarism), many of the countries by then were showing off their power in terms of military strength (building weapons and building up armies)
A (Alliances) = Most of the countries that went to war were because of a chain reaction of alliances formed designated to protect each neighboring/friendly country.
The allies were the United States, Britain, France, Russia, and Italy. The Central Powers were composed of Germany, Austria-Hungary, Bulgaria, and the Ottoman Empire. Basically, one threat led to another which led to the countries flaunting their power, which then led to war, then to their allies helping out with their personal situations.
I (Imperialism) = many countries strove for power, this meant capturing and keeping hold of smaller nations for gains and resources (so this was a constant competition to see who could hold what)
N (Nationalism) = Most people were flared up about their country (boasting it and declaring it could defeat the others) so again, this led to the flaring/flaunting of country powers (aka patriotism)