The Giver tells Jonas he can’t escape with him because he will be needed to work with the people to help them deal with the returning memories.
When Jonas becomes the new Receiver of Memory, he has no idea what to expect from this assignment. He does not even know what he is supposed to do until he begins training. Then Jonas begins to be entrusted with the collective memories of the community, from way back through time. He starts to see that things have not always been the same.
Jonas learns about snow, for example, when he takes a sled ride.
"And it came from the sky." "That's right," the old man said. "Just the way it used to." "Before Sameness. Before Climate Control," Jonas added. (ch 12, p. 85)
Not all memories are peaceful. Some of them contain great pain, and this is why they are hidden from the community.
Jonas’s community exists by a principle called Sameness. It means that there is no individuality, and a person’s choices are made for him. There is no color, and there are no emotions. No one stands out in any way. At first, this makes sense to Jonas because it is all he knows. Then he realizes that there are problems.
When Jonas sees his father kill a newborn by lethal injection, just because it was the slightly smaller of a set of twins, he is devastated. He and The Giver decide that Jonas must escape, retuning the memories to the people, for things to change and humanity to return.
Jonas asks The Giver to go with him, but he knows what the answer will be.
"My work will be finished," The Giver had replied gently, "when I have helped the community to change and become whole. (ch 20, p. 162)
Jonas understands that the people will suffer when the memories are returned. They will be lost and confused. Pain is part of the human experience, but they have never had it before. Jonas and The Giver realize that in order for the people to feel love, they need to feel pain. Yet they will need The Giver to help them through it.
Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book). Houghton Mifflin Harcourt. Kindle Edition.