Why was Paul unable to relate to his family and community when he went on leave?

Paul feels distanced from home life, emphasized by the civilian clothes he has out grown. He cannot relate to any civilians, most of all his family. His dealing with Kemmerich's mother is a more pronounced version of how he relates to his own mother; he protects her from the painful truth. Paul's mother, however, sees through him more clearly. She understands she may never see her son again. The irony, though, is that it may be from her death from cancer, not his from war.

Remarque, one must assume, also felt he could not relate to civilians or put his experiences into words. Nevertheless, at some point he decided that the war was either too important to ignore in writing, and that civilians might understand more than "half" of what soldiers feel. Perhaps, too, he felt that writing was the only way for him to confront and, possibly, master the terrible experiences.