Jonas does not like telling his dreams because he almost never dreams, and when he has one it is a sexual dream about Fiona.
Jonas rarely dreams, but after he helps Fiona at the House of the Old, he has a vivid dream about trying to get her into a bath tub.
He paused. He knew he had to tell it all, that it was not only all right but necessary to tell all of a dream. So he forced himself to relate the part that made him uneasy. (ch 5, p. 36)
Jonas’s parents realize that this is the first of the Stirrings, or sexual feelings. Jonas only has to take a pill, and the feelings and dreams will go away. All community members take these pills, because the population is tightly controlled and all children are planned and artificially conceived. It is interesting that in a society that never mentions sex, Jonas is still embarrased to discuss it.
Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book). Houghton Mifflin Harcourt. Kindle Edition.