Jonas is more mature and more thoughtful by the middle of the book because he has been through some of his training as Receiver of Memory. Through the memories, he has learned enough about his community to realize that there is a better way. It makes him curious about what else he does not know.
By this point in the book, Jonas is learning about himself as he learns more about his society. He realizes that he had no idea what his community was really all about. He had always accepted it as it was. He thought of it as a perfect world, because that was what he had been raised to believe.
When Jonas was younger, he thought that Sameness ensured that everyone in his community would be happy. He also did not see anything wrong with everyone dressing alike and being told what to do all of the time and how to think. After he began his training as Receiver of Memory, he realized that there was something wrong with this way of life. He came to understand, for example, that his people did not feel the full range of human emotions.
Jonas is surprised when The Giver tells him that the people of his community “know nothing.”
"It's just that ... without the memories it's all meaningless. They gave that burden to me. And to the previous Receiver. And the one before him."
"And back and back and back," Jonas said, knowing the phrase that always came. (Ch. 13)
The more Jonas learns from the memories, the more he realizes that The Giver is right. His people cannot think for themselves, or feel real emotions. He sees hunger and death in the memories, but he also sees love and real happiness. He comes to understand that there are benefits to letting people experience the full range of life experiences.