The main idea, or central theme, in Beverly Cleary's Dear Mr. Henshaw is the need to overcome life's obstacles, such as the divorce of one's parents, by developing a positive attitude.
Throughout the story, Leigh must face many problems. One of his biggest problems is that he doesn't want his parents to be divorced. An even bigger problem is that, as a result of the divorce, Leigh feels very lonely because he is home alone a lot and feels cut off from his father, who breaks his promises that he'll phone Leigh. Leigh gets so upset by his father's broken promises that he feels forgotten and unloved. Aside from family troubles, Leigh is unhappy at his new school because he has no friends and is being bullied by someone who steals from his lunch. At the turning point in the story, called the climax, when Leigh complains to Mr. Fridley about not having any friends, Mr. Fridley gives the following reply:
Who wants to be friends with someone who scowls all the time? ("Tuesday, February 6")
Mr. Fridley also underscores the central theme of the story by further pointing out that Leigh is so wrapped up in his own problems that he's forgetting to notice everyone has problems:
So you've got problems. Well, so has everyone else, if you take the trouble to notice. ("February 6")
Because Leigh is only seeing his own problems, nobody else's, he feels isolated from the world when he isn't truly isolated. Mr. Fridley also gives Leigh advice that helps create the turning point in the story and further underscores the central theme:
You gotta think positively. ("February 6")
Once Leigh begins thinking positively, he finds a solution to try to catch the lunch thief by rigging a lunchbox with an alarm. While the alarm doesn't help him catch the thief, it does help him see that the whole school has been having trouble with the lunch thief, not just him, which makes him feel connected to the world and not so lonely for the first time in a long time. He even begins to make friends because of his positive thinking and forgives his father for letting him down, while accepting both his father's good and bad qualities.