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I think the relation between the quote and the themes is fairly simple. Its the freedom of choice! Jonas chose to give some of his memories to Gabriel, regardless of what the rules say. He made the choice to let Gabriel into information, he would not have believed if he was Jonas' age! But since Gabriel is only a baby Jonas can tell him things and share his memories with him, and Gabriel can't even say he is crazy because he had seen the memories and he is just a baby! Jonas has the freedom to lie to people! At first he thought that was ridiculos, but soon he realized that its actually very helpful! He can lie to The Giver about not sharing his memories! So this quote is all about the themes of freedom and choice!
Information and Images from:
http://www.isklmrthomas.blogspot.com/
https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiYfUXC-UgCWs2UF-43UZT_-AKHVHYahqUTGdXLzWJy1pk09nVzL8U9abwLZswgsGE3AGLqGGbFUo_pScBq5AWNVTCKudaDOg-Jj8iK9PETn9iuIsBpYDFogTd0PKVIcykHm2kNSnBNOCk/s400/the+giver.jpg