Stanley lied in order to spare his mother pain and worry.

In the story, we learn that Stanley has been sent away to a camp for "bad boys." At Camp Green Lake Juvenile Correctional Facility, Mr. Sir tells the boys that they are responsible for digging one hole each, every single day.

"You are to dig one hole each day, including Saturdays and Sundays. Each hole must be five feet deep, and five feet across in every direction. Your shovel is your measuring stick. Breakfast is served at 4:30."

On Stanley's first day, he begins to realize how difficult it is to dig a hole that is five feet deep and five feet across in every direction. The surface (about eight inches deep) is the hardest to break through. By the time Stanley breaks through the surface, he has a blister on his right thumb. This makes it painful for Stanley to hold on to the shovel.

Unfortunately, the worst is yet to come. By the time Stanley is finished, his blisters have broken open, and his hands are openly bleeding. Every part of his body is aching. At the end of the day, Stanley is so exhausted that he can only stand still as he showers. He is too tired to lather up with soap.

So, this is what Stanley tells his mother when he writes to her:

Dear Mom, Today was my first day at camp, and I've already made some friends. We've been out on the lake all day, so I'm pretty tired. Once I pass the swimming test, I'll get to learn how to water-ski . . .

Stanley knows that he is lying, but his primary goal is to spare his mother pain and worry. From the text, we can infer that Stanley loves his mother and has a reasonably good relationship with her.

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.

Why did Stanley decide to lie to his parents about what he is doing at camp?

In his letter to his mom, Stanley lies about what life is like at Camp Green Lake. He doesn't tell her how hard it is; how exhausting and back-breaking it is to spend your whole day out in the hot, baking sun; how he's so physically wrecked at the end of a day's work that he can barely stand up in the showers. Instead, in his letter home, Stanley just says that he's "pretty tired." He also says that he's been making friends and that he'll get to learn how to water-ski once he's passed his swimming test, his parents not knowing that Green Lake is dried up—no longer a lake at all.

Stanley won't level with his mom because he doesn't want her to worry. If he told her the full story then she'd spend every minute of the day fretting about her son, and Stanley doesn't want that. We can infer from this that Stanley has a good relationship with his mom, which is more than most of the boys at Camp Green Lake can say.