In the book, Jonas was able to feel pain and to feel the suffering of war because the Giver gave him those sorts of memories.
The first negative memory that the Giver gave Jonas was a painful one. It was a memory of a sunburn. That shows us how little real pain anyone ever felt in the community. Jonas had never felt anything that bad before.
Later on, the Giver gives Jonas the memory of a battle. Jonas occupies the memories of a young soldier who is dying in the battle. But after the soldier dies, Jonas continues to experience the horrors of the battle.
In the book The Giver Jonas felt pain and the suffering of war through the memory that the giver had passed to him. The event was very traumatic as Jonas was experiencing the death and suffering of a young soldier on the battlefield. He looked around him and could feel the intense pain and fear that accompanied the soldier's thoughts.
The experience was also more severe because Jonas had no previous concept of death either. Later when he saw his friends playing war he is horrified by their game. He tells them to stop. The memory is too real for him.