Answer:
Paul reacts coldly, because he starts to see people empty. He reacts quickly, because he cannot tell people the brutality he has gone through. He acts strangely because he no longer identifies with those people.
Step-by-step explanation:
Paul received very heavy and rigid military training and witnessed the horrors of war. He started to see the concepts of patriotism and national responsibility as a cliché and illusory thing, because in the war, patriotism is useless and only terror is visible. After going through all of this, he gets a leave to return home, but he is not the same person as before. He no longer identified himself with those people, he felt sad and repressed for having to hide the things he witnessed and for finding them empty.