What did Jonas give to Gabriel and why did he give it to him? The book 'The Giver"

Jonas gives Gabriel part of a memory that he recieved earlier in the day. It was a tranquil scene of a sailboat in a bay, and Jonas gave it to him to help calm Gabe, so he could relax and go to sleep. It became a problem because Gabriel often gave him these memories and when Gabe was taken away from Jonas he couldn't sleep through the night.