I would want to argue that the key theme of this novel is the way that the community of which Jonas is a part has stripped away all decisions and emotions from human life and how negative this is. This theme is of course highlighted by the way in which Jonas, as he develops through the novel, becomes more and more criticial of the society of which he is a part. Consider, for example, what we are told about how his character changes in Chapter Thirteen:

He found that he was often angry, now: irrationally angry at his groupmates, that they were satisfied with their lives which had none of the vibrance his own was taking on. And he was angry at himself, that he could not change that for them.

Jonas is experiencing something of what life could actually be like in a world free from the control of a totalitarian state that allows its populace no free will or decision-making powers. Consider too the experience of Jonas when the Giver gives him the memory of love and how this is a concept that Jonas finds incredibly attractive and compelling. The development of the character of Jonas is therefore used to implicitly criticise the community of which he is a part and which has stripped humans of everything that defines them as humans.