Why does Leigh's attitude change by the end of Beverly Cleary's Dear Mr. Henshaw?

In Beverly Cleary's Dear Mr. Henshaw, Leigh's attitude changes toward the end of the book due to excellent advice he receives from Mr. Fridley, the school custodian.

By February, Leigh is so brokenhearted by his father's treatment that he feels he has to take it out his anger on someone. He tries to seek revenge on the lunch thief by kicking any lunch he comes across down the school hall. Mr. Fridley stops him and says, "I don't want to see a boy like you get into trouble, and that's where you're headed" ("Tuesday, February 6"). When Leigh responds by complaining about not having any friends, Mr. Fridley tells him he "scowls all the time," which isn't very friendly, and tells him he is so wrapped up in his own problems that he's forgetting to notice everyone around him has problems. Mr. Fridley then says, to overcome Leigh's problems, he's "gotta think positively."

It is at this point in the story that Leigh begins to think positively. First, he is made happy when he sees swarms of beautiful monarch butterflies in a butterfly tree park. Then, he thinks positively by rigging an alarm inside of his lunchbox. The rigged lunchbox fails to catch the thief but helps Leigh make friends because the whole school is very impressed with his invention.

He even begins thinking positively about the lunch thief because he no longer cares to find out who the thief is; he realizes he no longer wants to judge the thief:

Maybe he was just somebody whose mother packed bad lunches—jelly sandwiches on that white bread that tastes like Kleenex. Or maybe he had to pack his own lunches and there was never anything good in the house to put in them. . . I'm not saying robbing lunches is right. I am saying I'm glad I don't know who the thief was, because I have to go to school with him ("Thursday, March 15").

Leigh also develops a more positive attitude about his father. Though he still misses his father and feels hurt, he has come to accept that his father will disappoint him and cannot be relied on. He has also begun to feel compassion for his father out on the lonely road, which is why Leigh tells his father he can keep Bandit once Bandit has been found and returned to Leigh.