imagine being told that your enemy has only sticks and you get paid 10k dollars per month for just pressing a trigger and gunning them down. of course this might be a bit overblown but the point still stands. people do it for money. because to them it sounds like easy money.  other situation is when you have the choice of either fighting or getting 10 years in prison/tortude/death with barrier troops. then you go to war but unwilingly. basically its either because people want money or they get punished hard for it to the point of dying in war would be preferable choice.
Germany