There are two main reasons that Jonas is careful with his word choices. The first is that in the Community, words have very specific meanings, and so using the wrong word is inefficient and causes people to waste time and energy. Instead, children are educated to always use the right words so that their later lives will be lived to the utmost efficiency. Jonas has witnessed the various problems caused by incorrect word choice, and so is careful to always use the right words in expressing himself.
Now, thinking about the feeling of fear as he pedaled home... it was not what he was feeling now with December approaching. He searched for the right word to describe his own feeling.
Jonas was careful about language.
(Lowry, The Giver, Google Books)
The second reason is that using the wrong word is often grounds for punishment or even "Release," which Jonas discovers means euthanasia. People in the Community who use incorrect language cause significant problems with the structure and scheduling of the society, which is tightly controlled, and so if he continuously fails to use the correct words, Jonas runs the risk of being an outcast or being killed. Correct word choices allow Jonas to function within the society even as he learns about the real word outside, and the real meaning of his feelings.