Before he is given his Life Assignment and before he is chosen as the new Receiver of Memory, Jonas has a normal relationship with his friends and family. He is close and friendly with everyone and lives a rather mundane and boring life, blindly following the Community's rules and regulations.
After he's selected as the Receiver of Memory and becomes capable of recognizing and feeling emotion, Jonas realizes that he can no longer be as close to his friends and family as he used to be, because the relationships he shared with everyone weren't actually genuine; the fact that they all liked and respected each other was simply because society told them that that's the way things are supposed to be, not because they truly felt love or affection for one another. Jonas confirms this when he asks their parents if they love him and they laugh at him and tell him that he used an old, "generalized word, so meaningless that it's become almost obsolete."
During his training with the Giver, Jonas learns that being a Receiver of Memory is a very lonely and difficult job, as he has to keep all of the memories and emotions, as well as his training, a secret from everyone else. Even if he tries to interact with his parents or his peers in the hopes of getting a genuine reaction out of them, he knows that he will fail, as they simply do not know how to feel and don't know the reality of the situation as he does. They don't understand what's truly happening, and they're content with Sameness, and Jonas can't do anything to change that.
Thus, Jonas closes himself off and becomes more distant; he's no longer as open with his parents, and his friends think that they have to respect and like him because he's the Receiver, which is a very honorable job in the Community. He can no longer play war games with Asher, for instance, because he now knows how horrible actual war is and doesn't wish to be reminded of the pain and suffering.