Why was World War I a different experience for a British or German citizen as opposed to an American citizen?
World War I was a different experience for these different citizenries mainly because the British and the Germans were involved in the war from the beginning and the United States was not. This meant that the people of the two European nations were much more dramatically impacted than those of the United States.
To see how the impact was different, let us simply look at casualty statistics for the war. The United Kingdom lost a bit over 2% of its population in this war. There were over 886,000 military deaths and 107,000 civilian deaths over what would have been expected for that period. Germany was somewhat harder hit. It had over 2.4 million deaths. Since it was a bigger country, this only came out to 3.8% of its population, but that is still a huge number. A further 4.2 million Germans and 1.6 million British were wounded in the war. In contrast, the US incurred very few casualties since it was in the war such a short time. The US lost 117,000 people. This is a large number of people and the emotional toll would have been great. However, this is only .13% of the US population. American wounded were a further 205,000.
What this means is that Britain and Germany suffered something in the range of 20 or 30 times more casualties than the Americans did. This means that the average Briton or German would have been much more likely to have been personally impacted by the war than the average American.
Because the war went on so much longer for the Europeans, and because they lost so many more men, the impact on them was much greater than the impact on the US.