Why does Stanley lie to his parents about what he is doing at camp, and what motivated him to do it?

In Louis Sachar’s young adult novel Holes, Stanley Yelants is sentenced to Camp Green Lake: a camp for troubled boys. Stanley explains that the judge offered him the options of jail or camp, so he picked the camp, since he’d never been to a camp before. However, when he arrives, he quickly realizes this isn’t a typical camp. To begin with, Camp Green Lake has no lake. Stanley meets Mr. Sir, who explains the rules of the camp: the boys will get up every morning at 4:30 and go dig holes. He warns Stanley, "If you dig up anything interesting, you are to report it to me or any other counselor," but he doesn’t give any further explanation.
After the first day of digging, Stanley, now called Caveman by the boys, writes his mom a letter about camp. He fills the letter with lies about swimming and making friends. He explains to Zero that he lies because he doesn’t want her to worry. Later, he writes her again, still lying to protect her. This time he to fills the letter with made-up stories about rock climbing so that she believes that Camp Green Lake is a normal camp and that he’s having a great time.