What did the population of Germany feel about joining WWI?
In Germany, as in practically every other country to participate in World War I, there was a great deal of enthusiasm for war at the time that the war started. People were told that the war would be very short and that it would be a very good thing for the country.
According to one important book about WWI (The Guns of August by Barbara Tuchman), the German papers were ecstatic about the war. One paper said that it was "a joy to be alive" at such a time. Another said that Germans were "exulting with happiness" because the war had finally come. It said that Germany had "wished so much for this hour." (All quotes are from page 145 of the paperback edition of the book published by Dell Publishing in 1963.)
Of course, not all Germans were happy about the war. The leftists, in particular, were unhappy. They felt the war was going to be fought by the poor for the benefit of the rich. However, the overall public opinion was strongly in favor of going to war.