Money. The Germans had finally become a formal nation and wanted their peice of the pie, the English and French didn't want to give it.
Furthermore, in England, it was partially a move by the remnants of the feudal class to reconsolidate their power after the Parliment Act shifted the balance of power to the House of Commons.
The War Office then was able to supercede the parliment. This was an almost inevitable outcome since after the Curragh mutiny the government could not depend on the armed services any more at all and everything was to be ripped assunder.
For the Tsars it was something of a Holy War, as they viewed themselves as the legitimate heirs to Rome - which would mean that it was an excuse to take Istanbol and all that crap. Most of the Russians didn't care about this, however.
I can't say for the French, I don't know enough about it - aside from a system of alliances and a historic rivalry with the Germans.
For the vast majority of Americans, at the start they wanted the Germans to win as - most of them were of German descent; the second greatest number were militantly Irish and thus anti-english; and at the time, England was America's historic foe for the Revolution, 1812, arming and advising the Confederacy, and a few other things.
However, Wilson, in addition to being something of an Anglophile, knew that much of the US dollar was invested in England. If the investments were to be paid back, England had to win. Secondly, both the US and Germany were founding new colonies and whatnot at the time and were starting to get in each other's way. Thus, Wilson worked with the English to coax the public in to the war.
But I can say this, and I think it's true:
The First World War was the Old World's Dying gasp - in all countries involved the last feudal remnants were almost powerless at best, or outright dead, by war's end. Everyone began to organize for the people's greater good.
The Second World War was the birthing pains of the New World - which system, which style, would set the standard for Earth.
Alis Volat Propriis; Tiocfaidh ár lá; Proletarier Aller Länder, Vereinigt Euch!