The community controls the Stirrings to keep people in continual preadolescence. People are given pills as soon as Stirrings occur, and the pills block the hormonal change. This prevents non-approved population and keeps the citizens docile.
Stirrings is a euphemism for puberty. The community carefully monitors children who are about to enter puberty to prevent it from happening. At the first signs of puberty, children are given a medication that prevents the Stirrings.
Jonas’s parents are first aware that he is entering puberty when he has a dream of a sexual nature. The dream shows that Jonas is beginning to have feelings for the opposite sex. This is when the Stirrings pills start for each child. For the community, taking the pills is just a normal part of life.
Jonas’s mother explains the procedure for taking Stirrings pills to him.
"But you mustn't forget. I'll remind you for the first weeks, but then you must do it on your own. If you forget, the Stirrings will come back. The dreams of Stirrings will come back. Sometimes the dosage must be adjusted." (Ch. 5)
It makes sense to control the population by preventing people from wanting to have sex, something that would create unwanted or unplanned babies. The community has carefully planned each birth. It makes sure the child will be genetically appropriate and that only a set number of babies are born each year. This way there can never be overpopulation.
There is another reason for the Stirrings pills though. They control hormones, but they also seem to control adult thinking. The citizens of the community are basically kept permanently immature. The pills limit their access to developments such as rational thinking and emotions. People’s emotions are kept in check by the Stirrings pills.
When Jonas goes through his training, he learns about the emotions that most people had in the days before Sameness. As Jonas learns about the reality of human life, he stops taking his pills for Stirrings.
He had not taken the pills, now, for four weeks. The Stirrings had returned, and he felt a little guilty and embarrassed about the pleasurable dreams that came to him as he slept. But he knew he couldn't go back to the world of no feelings that he had lived in so long. (Ch. 17)
Jonas is aware that no one in his community is capable of a genuine emotion, other than The Giver and now him. The Stirrings pills are obviously part of the reason why. If you keep people immature and unthinking, they will never question anything. It is the cost of perfect obedience and docility.
Although population control is a worthy goal, the community’s method of isolating and preventing emotions in its people is tyrannically abusive. No one in Jonas’s community is a real, whole person. No one is ever an adult. They are more like drones or robots, completely unaware of what is really happening to them. Jonas’s community is not one without pain; it is one where pain is hidden away and the people are kept too drugged to understand it.