Why is Paul Baumer so angry at Kantorek?

Paul has returned to his village on leave. There he finds his old teacher Kantorek extolling the virtues of war to young impressionable students. Paul once was such a student believing in the "Iron Youth" and Kantorek's other rubbish. When Kantorek asks Paul to tell his students about the glorious war, Paul replies,

"We used to think you knew. The first bombardment taught us better. It's dirty and painful to die for your country. When it comes to dying for your country it's better not to die at all! There are millions out there dying for their countries, and what good is it?"

This is not what his old teacher wanted to hear. Paul's bitter disillusionment is finally heard by the boys and his former teacher.