Before Jonas is selected, the Giver had been the Receiver. This meant that he received all the memories of generations and generations before him. There were memories that were happy and beautiful. But there were also memories that were sad or painful or ugly. These are what the Receiver has been burdened with since he was selected at the age of twelve, the burden of all the world's memories. Once Jonas is selected as the Receiver, the Receiver becomes the Giver. What makes the Giver sad is that he understands what a burden it is to carry all the memories of the world, and it pains him to have to now have to pass this burden on to Jonas. He realizes that he will be relieved of these memories, and that will be good for him, but he is an unselfish man, and he knows that much of what he is passing on to Jonas will be painful and unpleasant. In the Community, no one experiences any pain or unpleasantness, and Jonas is ill-prepared to receive all of this woe. It is natural that the Giver would feel sad about this.