Most men were so eager because at this time countries were pushing independece and "only the strongest will survive". With a new way of thinking, instead of uniting all of Europe and now becoming their own nations, men were eager to fight who they were. They were told this would be the war of the century and no one wanted to miss out.
Soldiers weren't properly trained with any of the new weapons created during the industrial revolution. Very few were actually trained properly. Also, the trenches were nothing like what they had imagined. Little food, water, excessive lice and diseases. No soldier was prepared for the living conditions they were put in.
Finally as the war came to an end, US President Woodrow Wilson proposed some 14 points that soon changed Europes map forever. American troops steped up and brought a bigger defense which helped end the war. Germany decided to quit fighting, however, they would try again at a later time...