Why the did attitudes to the war of soldiers in world 1 change by 1917?

Before WWI there was the popular idea of a code hour heroism all of the cliches that are still being bandied about today in relation to war. People had no idea how terrible a situation could be. especially when taking into account the fact that the war is being fought to protect the interest of an upper class that seemed to have an almost insatiable thirst for more of the blood of the ordinary man. People just became tired of the constant terror and pain involved in this conflict.