Why do you think so many young men volunteered to fight for their country at the start of world war i ?

Oh that's simple. A lot of women at the time were treated very differently from men, they were literally bullied. They were told that they are weaklings and that they can't do their work and all that nonsense. Then, when the men were at work, the women all had the opportunity to prove themselves, and they really did a splendid job. 
At the same time, some young women also volunteered that they should go fight in the war, too. Saying that they were capable and all that. Their main motive behind this move was to prove to all those men that women are also something great - just like men. 
We should appreciate women and men and we shouldn't ever create differences between them.

I'm sure many of them volunteered because they decided it was the right thing to do. By winning the war they could make everything better fir their children, and all their friends children.

Many more would've volunteered because of peer pressure, maybe their families urged them to fight, or their friends were going and they didn't want to be left behind.