Answer:
As nationalism and imperialism were on the rise, the competition for resources, land, and power was at play. Many European powers strived to become the top military and economic top dog so to speak. Britain had been the lasting empire with the most territory, Germany had been an emerging power with its brute military force and economic development, The French Republic was still sour about the Franco-Prussian War and demanded the territory of Alsace-Lorraine back, Austria-Hungary was a duel monarchy with multiple ethnic states inside its borders protesting for independence and the Russian Empire had been an undeveloped nation with political instability. What made these countries come together to fight one another? Their own interests and alliances. The British Empire was satisfied where it stood as number one in Europe but the invasion of a neutral Belgium by their rival Germany had crossed the line and soon enough declared war on them. Germany out of its own interests wanted to finally destroy France once and for all also to expand their colonial empire to rival that of Britains. Austria-Hungary wanted to keep order and stability in their empire and put down independence movements in the Balkans that threatened Serbia. Russia in fact had wanted its own military pride as the monarchs were slowly dissolving into a hole of chaos and they chose to back Serbia due to Slavic heritage.
So the reasons why the war was good for these nations were out of their own interests and the reason they created alliances with each other because they knew that country would not interfere with their interests.
