There were various reasons why people, especially young men, chose to join the army during the first world war, including feelings of patriotism, a desire for adventure and other motives. Many young men saw joining the army as an opportunity to see the world. They may have also been spurred on by friends and family.
At first, joining the army was not seen as so dangerous a profession, particularly because people believed the war would be over quickly. As the stalemate developed and the death toll rose, however, men and women that enlisted into the army joined for more sobering reasons. People were also persuaded by the many propaganda posters that existed during the time. These posters depicted service in the army as a noble cause, and were especially popular in the US. Eventually, many countries also resorted to conscription, forcibly drafting people to serve in the military as the war waged on and incurred drastically high casualties.