Wow, joxy, that is really a question and a half. Whole books have been written on the subject, so I can't do much more here than give you a few general causes.
- The assassination of the Austrian Archduke Franz Ferdinand. This ultimately led to the demands Austria made on Serbia that, when not totally accepted, triggered an invasion (and started WWI.)
- The system of alliances, by which Europe was an interlocked net of countries that "had each-other's back." It went a little something like this: Austria declared war on Serbia / Russia and France get ready to declare war on Austria / Germany declares war on Russia and France first (to get a jump on them and help their buddies the Austrians) / the UK declares war on Germany (because Germany, to get at France, rolls through neutral Belgium and won't get out) / Turkey jumps in on Germany's side / Italy (though allied with Germany) decides to backstab it by joining the "allies" / and finally the Americans stomp there way in to seal Germany's fate.
- New technology and a massive arms build up that gave nations an itchy trigger finger.
- The notion that wars were often fought, won, and lost quickly and without significant bloodshed.
There are a lot of other, more complex reasons, but these are really the big ones. Basically, everyone thought they could win quickly and everyone had a lot of big friends to bring to the party. In the end, there was a whole lot of dying, the re-working of Europe's map, and a bitterness that let to World War I part 2 about 20 years later.
There were many people and events that contributed to the beginning of WWI, but I don't have enough space to go into all of them. Generally, there are four main causes of the Great War.
- militarism: The major powers of Europe began building up their militaries to assure that no other country became more powerful since this would make them vulnerable to attack and/or loss of territory;
- alliances: In an effort to maintain a balance of power in Europe, countries made agreements with each other that they would come to each other's defense if one was attacked;
- nationalism: This was an intense feeling of loyalty to one's country that led the people of a country to believe they were united as an ethnic group and were better than people of another country;
- imperialism: Thiis was the rivalry between nations to control another country and reap the benefits of that country's resources
This brief overview gives you the basics of the causes of WWI. Go to the links below for much more information.