When World War I broke out in August 1914, there was a surge in young men coming forward to volunteer to fight. There were many reasons for this:

Patriotism - people supported their country and wanted it to win.
Propaganda - the government advertised for volunteers telling people that their country needed them.
Expectations - they thought that it would be a short war that Britain would win. They weren't aware of the horrors that were to come in the trenches.
Excitement - most people had never been abroad before.
Gender ideas - many men believed that it was their duty to protect women and children.

To increase the number of volunteers, the government also introduced Pals Battalions [pal is an old-fashioned word for friend]. These were units of soldiers who would be drawn from the same village or work-place so that they could go to war with friends. Sadly, this led to some villages losing a huge proportion of their young men if a Pals Battalion was in a part of the front-line that was particularly badly hit, so the Pals Battalions were broken up in 1916 after the horrendous casualties at the Battle of the Somme.

From 1916 the government introduced conscription which meant that young men were forced to fight.

Year 7 Daily Life and Beliefs

Why did so many sign up to fight in WWI?