In the Community that Jonas lives in, every aspect of human life is very tightly controlled as a means of keeping people safe from the pain of having unpleasant experiences, emotions, thoughts, etc. Reporting one's dreams or any feelings after one has them thus becomes a critical step in monitoring the attitudes and thoughts of members of the Community. On the surface, speaking of these dreams and feelings helps people "deal" with them and is a component of the honesty policy of the Community; more secretly, it helps the Elders of the Community determine whether or not someone's medications need to be adjusted in order to suppress those feelings.
Thus, when Jonas experiences "stirrings" (the first sign of budding adolescent sexual attraction) in a dream he has about Fiona, Jonas is obligated to share this dream with his family and to articulate that the strongest feeling in the dream was a sense of "wanting." These hormonal changes are quickly cut short through a dosage of pills.
Jonah's world is based on everything being safe and the same. All of the memories and emotions are only carried by The Giver. People have to report their dreams and share their feelings to make sure no one has any negative dreams or feelings and to stop them early if they do. So when Jonah reports the dreams that signify his "stirrings" they can give him medicine right away to take away the stirrings and the emotions they cause.