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This is only a partial answer:
Even when conscription wasn't a factor there was enormous social pressure to enlist. Those who refused were accused of cowardice. See for example the Order of the White Feather, a movement of civilian woman, often young and attractive, who were encouraged to present a white feather to any man of fighting age seen in public not in military uniform. The white feather was a public accusation of cowardice. The order was started in Britain at the outbreak of WWI by Admiral Charles Fitzgerald and was so successful and so zealous that it was necessary to provide civil servants and honourably discharged soldiers with badges so that they weren't publicly shamed. It also spread well beyond Britain.
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Every nation had its own "pet peeve" which had to be addressed.
As you mention yourself, the common expectation was a quick triumph, and such atmosphere is quite conductive to militant patriotism flaring up.
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In both France and Germany there was mandatory conscription, so both countries had as large an army as they could afford. It was a crime to fail to enlist if you were ballotted into the army. Both countries publicly glorified the army and soldiers as patriotic entities. Also, the pay was better than many starter jobs available to young men at that time.
Britain's army was entirely voluntary until 1916. The government used patriotic propaganda and organized recruiting to enlist soldiers. A typical technique was to enlist young men from particular companies, areas or schools all at the same time, creating a sort of peer pressure for the groups to enlist and fight together. As in Germany, patriotic fervor was a prime motivation in addition to the pay.
The United States generated a range of patriotic propaganda to encourage enlistment, but this was completely insufficient, so that when the US entered the war it was necessary to forcibly conscript soldiers. For example, in 1916 the president appealed for 1 million volunteers and less than 100,000 enlisted. Therefore, after war was declared in April 1917, it was immediately followed in May by compulsory conscription and it was made a crime not to report to the army when summoned. By this time all the other major belligerents had done the same thing.
As the war dragged on, both enlistment and desertion were serious problems to the extent that mass executions by firing squad were carried out on both sides. Robert Graves wrote about his experience in his famous book, Goodbye to All That, "I had my first direct experience of official lying when I arrived at Le Havre in May 1915 and read the back-files of army orders at the rest camp. They contained something like twenty reports of men shot for cowardice or desertion. Yet a few days later the responsible minister in the House of Commons, answering a question from a pacifist, denied that sentence of death for a military offence had been carried out in France on any member of His Majesty's Forces". Note that this was just one rest camp, out of hundreds, and that was in 1915, before conscription had even started.