Let us remember for one moment what Jonas is sacrificing. He is giving up home, all that he has ever known and a feeling of security, for a completely unknown existence outside his community, where he has never gone before and which he has no knowledge of. He is defying the rule of his community openly, which he has only done in secret before. These are massive issues for a boy of Jonas' age to face. To be forced to abandon your home and family so that you can stand up for what you think is right is a big decision to have to make at the age of thirteen, and yet Jonas does it without even thining about it too much. Note how his feelings are described as he leaves in Chapter 21 of this great novel:
He felt, surprisingly, no fear, nor any regret at leaving the community behind. But he felt a very deep sadness that he had left his closest friend behind.
Perhaps his lack of fear and regret reinforces the sense that Jonas knows he is doing the right thing by leaving to save the life of Gabriel. His courage in standing up for what he believes to be right and then acting on it by leaving his home is self-evident.