You can find the answer to this towards the end of Chapter 17.
Seeing his friends Fiona and Asher and the other kids playing the war game totally freaks Jonas out. It makes him feel terrible, so much so that he just about starts crying.
The reason for this, of course, is that he (unlike any of the other kids) has actually experienced what war feels like. He has lived the experience of the soldiers who were wounded and dying in the war in the memory the Giver gave him. Because of that, he cannot enjoy the game the kids are playing.
Jonas, unlike all the other children in the community, understands what war really is. Since the other children have no conception of what death means, they are unable to truly grasp what war is. Jonas knows because he has received memories of war from the Giver and realizes that war means death, and that death means horrible pain for the one dying. It means loneliness, sadness, and grief for those still living.
Because of this, Jonas gets very upset when he finds the other children "playing at war". He gets choked up and tells the others to stop playing that game, that they don't know what they are doing. They respond by saying that it's just a game. Jonas knows better, though, and can't participate, so he leaves.
He is mad because they do not know what real war is
Hello thank you so much for the helpful answer ! :)