why should students agree to pay tution at all?
Because the German higher education system is massively underfunded. Also I see no reason why the state should provide subsidies to the wealthy and upper middle class. That's why I think their should means-tested tuition in Germany.
why should soldiers agree to get killed in Afganistan?
Because fighting in a UN approved war of self-defense after an attack on an ally is the right thing to do.