The Great War, a conflict to which no precedent existed, was one which harbored a dichotic response from those on its front line, with both German and British solider's experiencing a gradual shift in their attitude. Common between belligerents was an enthusiasm in 1914, with the outbreak of war existing as an opportunity for these young men to stretch their nationalistic muscle and fight a quick war of movement. Assumptions of swiftness and delusions of victory however faded over time, with the stalemate initiated at the Marne in September 1914 affirming another factor unique to this warfare. A stagnated war of attrition would alter the collective mindsets of soliders, with the implicated trench-based warfare only further exacerbating their discontent. The conditions in the trenches, characterized by de facto structures, terrible weather, rife illness and consistent uncertainty to name a few, contributed to a culture of resentment and depression. While some fell to suicidal and depressed tendencies, many carried on with the sustained hope of the looming 'knockout blow'. Additionally the lack of harmonization between the solider stock and their commanding officers, exemplified with the notion of the french Chateau generals of the allied front, led to feelings of resentment. Overall the attitudes of solider's deteriorated in line with the progression of the drawn out and demanding warfare, moving from naive excitment, to confusion, resentment, and in the case of isolated German units, eventual mutiny.
A bit long and not considerate of source inclusion, but you get the gist.