Jonas takes the pills for the Stirrings because everyone in his community is required to in order to prevent sexual feelings.
Jonas’s community has many rules, and they are all designed to make sure that everyone is comfortable and avoids any kind of emotion. Emotion is the enemy. While the community does use population control and some kind of genetic engineering that the book is not quite specific about, the main purpose of the pills for Stirrings is to prevent feelings of love.
Everyone in the community that is an adolescent or older takes the pills. Jonas first takes one when he has a dream about Fiona in a bath. His parents ask him what the strongest feeling in the dream was, and he says it was, “wanting.”
"Jonas," she said with a smile, "the feeling you described as the wanting? It was your first Stirrings. Father and I have been expecting it to happen to you. It happens to everyone. (Ch. 5)
The dreams are indicators of sexual feelings, so the community knows that the children are about to hit puberty. They begin giving the children pills to prevent them from fooling around. All adults take them too, until they are no longer necessary.
Of course, there is a problem here. The Stirrings pills do not seem to be preventing sexual feelings. They seem to be preventing any feelings. The community has other methods of reinforcing the rule against feeling (such as requiring apology for everything, and harsh punishments for the smallest infractions). Passion in general is not allowed, but love is kind of a gateway feeling. Without it, life has so much less meaning. After he has been training and getting memories for awhile, Jonas stops taking the pills. He realizes that he needs to feel.
Love is a strong emotion. In fact, it might be one of the strongest ones that human beings are capable of. It produces pain too, because when you love someone and the person rejects you, it hurts. Anyone who has ever had a boyfriend or girlfriend knows that! People who are in love fight. They break up. That is part of love. Love means passion, and passion means pain.
Somewhere along the line, the community decided to do away with any kind of passion. In order to avoid anger and suffering, they had to avoid love too. In order to avoid over-population, they had to institute population control. Win-win, right? You can kill two birds with one stone here. It’s called Sameness. Put everyone in one genetic basket.
Everyone in the community comes from a birthmother, and the same genetic code (somehow). Of course, there are a few variations. We know that Jonas has light colored eyes. They have not eliminated that genetic code, because they need it. Jonas is somehow predisposed to receive memories. He has the Capacity to See Beyond. Gabe and Katharine have the same eyes too, and all of them are receivers.
There are other genetic traits that the community has rooted out though. We know that they have deliberately done this, because The Giver comments that they are frustrated by their inability to eliminate Fiona’s red hair.
"We've never completely mastered Sameness. I suppose the genetic scientists are still hard at work trying to work the kinks out. Hair like Fiona's must drive them crazy." (Ch. 12)
I guess Fiona’s lucky that being born with red hair doesn’t get you released. Still, they do not like it because it makes her stand out. We cannot be different. Sameness for all!
All children in the community are raised until they are a year old (an arbitrary number, determined not by age but by when the ceremony is), and then assigned to a family unit. The family units are also arbitrary. They are foster families of two adults, a male and a female, who have no affection for each other. Their sole purpose is to raise the children. Because the babies are raised in an institutional environment from birth, they also do not learn to bond. No one bonds with anyone. Ever. When Jonas becomes Receiver of Memory, and learns about love, he asks his parents if they love him, and gets reprimanded for lack of “precision of language.”
"What do you mean?" Jonas asked. Amusement was not at all what he had anticipated.
"Your father means that you used a very generalized word, so meaningless that it's become almost obsolete," his mother explained carefully. (Ch. 16)
The community has eradicated the concept. It means nothing to anyone but Jonas and The Giver. With the pills for Stirrings, they have eliminated natural birth and the real family. There are no actual parents, and certainly no grandparents. The extended family that Jonas saw in the memory does not exist.
Jonas goes to Gabriel and tells him that things need to change. People need a choice, and they need love. What they are doing is not really living. Life without love is not life. Even at his young age, he can see that. He has gotten a glimpse into what the community has lost, and is about to see what it is really like. As soon as he finds out what his community does to people in order to create the conformity it needs so badly, he will no longer doubt that things do need to change, and it is up to him to change them.