Jonas lies to his parents because he realizes they can never understand what he is trying to ask them.
Jonas’s reaction to lying to his parents is complicated. When he first learns that he is allowed to lie, he is horrified. No one in the community lies. When Jonas sees the rules for his training and realizes that he is allowed to lie, he has mixed feelings about it to say the least.
He had been trained since earliest childhood, since his earliest learning of language, never to lie. It was an integral part of the learning of precise speech. (Ch. 9)
Jonas’s mind reels with the idea that he might be living a lie. He has never been tempted to lie, and he thought that no one around him ever lied. However, now he wonders if others have been given the same instructions, and it is a very disturbing thought for him.
Jonas is not tempted to lie for a long time, but as his training continues he starts to become more and more different from the people around him. As he receives more and more memories, he begins to have experiences that the others will never have. Jonas is not allowed to talk about his training, and he soon learns why.
After a particularly powerful memory, when he sees a family and experiences the concept of love, Jonas looks at his own family situation and realizes that he doesn’t really see the same thing there. His parents show something resembling caregiving, but not real love. So he tests them.
"Do you love me?"
There was an awkward silence for a moment. Then Father gave a little chuckle. "Jonas. You, of all people. Precision of language, please!"
"What do you mean?" Jonas asked. Amusement was not at all what he had anticipated. (Ch. 16)
His mother tells him that he used a “generalized” and “obsolete” word with no meaning, and asks him if he understands. He tells them that he does understand, but in his heart he does not. He knows that “love” is not a meaningless concept, and therefore not a meaningless word.
The reason that he lies to them is because he knows that there is no way that they can understand. There is no point in trying to explain to them a concept for which they have no understanding, and never will. He is talking to two people who have never experienced a true emotion in their lives, and talking about a real emotion, and one of the strongest human emotions. Trying to push the issue would just upset and worry them, which they can't handle, we know.
After this incident, Jonas decides to stop taking the pill for Stirrings. The Stirrings pill suppresses feelings. It is designed to prevent sexual feelings and prevent unwanted pregnancies, but Jonas must think that it affects feelings in general. He wants to be a full-fledged human being, and feel all of his emotions, unlike the automatons in his community.
This is one of the most powerful scenes in the book. In it, Jonas realizes that the world he saw in his memories really does not exist anymore. His community has managed to truly stamp out humanity. It exists only in him, in The Giver, and in Gabriel. When Jonas lies to his parents, he is acknowledging that he is not one of them, the citizens of the community. He is one of the real people who belong to that other world, the world of the memories.