Economic reasons of first world war?

Wars are generally good for the economy; governments spend lots of money on weapons and supplies. When soldiers leave their civilian jobs to go to war, other people have to take their place at work, so employment goes up. Also, there had not been any major wars in awhile; several countries had developed new weapons technologies in the meantime, and World War I gave them the chance to try things out.

Another reason--acquiring new territories by winning a war gives your government more money because of taxes. The loser generally pays for the war as well, so if your side wins you get more ground for people to live on--which means more taxes, more factories and other industries--which means more taxes, and the other side pays for everything. The Russian Empire could expand westward, regaining ground lost years before in other wars. The German Empire could expand both east and west, also gaining ground lost years before in other wars. The English Empire could also expand their sphere of influence in the Middle East and in Asia (against the Ottoman Empire).

Mind, England was fighting a defensive war, since they did not start it, but they still stood to gain if they won--which they did. As in the Middle East and all of the oil that is now in Saudi Arabia, Syria, Jordan, Iran, Iraq and in Georgia! Imagine the money that would bring any government today if they controlled the oil in all of those countries. There is also the African front to consider. Both sides, by mutual agreement, did not start fighting against each other in Africa. But most sides also had spheres of influence in Africa that, if their side won, would be greatly expanded. Which is exactly what happened as well.