How did Jonas react to his friends' game in Chapter 17?
Jonas reacts badly to the game that Asher and his other friends are playing. He stands in the center of the field in which they are playing, struggling to not cry, and then he asks Asher to stop playing the game. He recognizes that the children are playing a game of war, good guys and bad guys pretending to shoot one another. This is a game he would have willingly participated in before becoming the Receiver, but now he has insights and feelings that others in the Community are lacking.
As the Receiver, Jonas is no longer on any medication that represses his feelings, and he now holds the memory of war. He is able to feel true emotions, and he has experienced the pain and horror of war from the memories the Giver has given him. None of the children or adults in the Community are able to feel any true emotions, and they have no memories of their own that would allow them to understand how dreadful war really is.
He realizes that his friends do not understand this, and he gives up trying to persuade them to not play at war. He sees that with the drugs and no memories, they could never see what he sees or feel what he feels. This is an isolating and painful moment for him, and as he walks away, he is walking away from his childhood, his comfort, and his peace. The knowledge he holds now has brought him great beauty, but it is also bringing great pain.