What does Lieutenant Kotler tell the family about his own life? Why does Father suddenly decide that the discussions are inappropriate at the dinner table?

Bruno brings up that he hates learning about history and Father scolds him for it. Lieutenant Kotler offers that his father was a professor of literature at the university, but that he had left Germany for Switzerland in 1938. This information embarrasses Lieutenant Kotler and disturbs Father, who comments with suspicion, "Strange that he chose not to stay in the Fatherland" (146). Bruno is frightened by the tension but also enjoys Father speaking harshly to Lieutenant Kotler.