The children are taught to be careful about language so they avoid lying.
In the community where Jonas grew up, there is nothing more important than avoiding discomfort. Children are taught to conform to expectations from a very young age, with the consequences for not doing so getting more and more severe as mistakes are made.
Language is carefully controlled in Jonas’s world. The community seems to realize that language has power, and this is why they are so careful to control what people say and feel. This is why they have dream telling and sharing of feelings, and why they teach children about Precision of Language.
There are many examples of care with language. In the beginning of the book, Jonas struggles to describe how he feels about the upcoming Ceremony of Twelve, where he will be given his lifelong assignment. At the ceremony, the Elder shares how Asher accidentally said “smack” instead of “snack” over and over when he was three years old, until they beat him so badly that he stopped talking altogether.
After Jonas begins his training and experiences the concept of love, he asks his parents if they love him. They respond by scolding him for using imprecise language.
"Your father means that you used a very generalized word, so meaningless that it's become almost obsolete," his mother explained carefully. (ch 16, p. 127)
Jonas is shocked, because in his experiences with the memories he has learned that love exists, and is not meaningless. It is this realization that helps Jonas see that he needs to leave his community, and return feelings to them.
Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book). Houghton Mifflin Harcourt. Kindle Edition.