Shriver focuses on the relative importance of innate characteristics and personal experiences in determining character and behaviour, and the book is particularly concerned with the possibility that Eva's ambivalence toward maternity may have influenced Kevin's development. Shriver also identifies American optimism and "high-hopes-crushed" as one of the novel's primary themes, as represented by Franklin, the narrator’s husband, who serves as "the novel’s self-willed optimist about the possibility of a happy family."
Rationalization for Kevin's behavior is one of the central themes of the story: when asked the simple question "Why?" after the massacre, he responds that he is giving the public the excitement and scandal that they secretly crave. Only in rare instances does another side of Kevin emerge: in childhood when he becomes very ill, and later, just before he is transferred to an adult prison and is evidently nervous. Near the end of the book when asked for the first time by his mother "Why?," he responds, "I used to think I knew. Now I'm not so sure."