In world war 1, one of the main reasons it was caused was because countries had so much pride in themselves. (Nationalism) For example it began with britain and Germany, and Germany was beating britain in terms of its size of it's naval forces. Britain wanted to have the strongest and largest naval forces so it continued to expand. This in a way provoked Germany and war began.
Britain also really needed a strong naval force because they were an island country meaning they relied on it, when the war broke out, they had little army forces so they sought alliances. The many alliences between both sides of the war expanded greatly and this brought what seems like the whole world involved in this. (hence: World War)