Obviously, the key way in which Jonas changes and develops as a character is through his important Assignment as the Receiver for the community. He has already shown that he appears to be like any normal 11 year old in his community. We as readers note that he is perhaps more intelligent than others - he likes learning, thinks deeply about issues and sees things in different ways to the rest of his community. However, in a setting where conformity is prized, these differences are easily suppressed. Jonas himself doesn't like being different - look how he shuns attention after receiving his Assignment.
As his training begins, however, these characteristics that were established early in the novel cause him to become fascinated and absorbed by the memories he is given. Likewise the vividness of the memories seems to change Jonas - he becomes very aware of emotions in a way that others aren't and he pays attention to beauty and suffering and becomes loving towards his family in ways that other people in the community are not. He also becomes strongly attached to his new emotions and beliefs that he is developing. This results in intense feelings of frustration about various aspects of the community. Jonas' concern for others and his desire to see justice make him want to see changes in this community so that the cruelty of the community can be stopped but also so that the members can be awakened to the joys and richness of life. Note the following quotes:
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.
You can see here Jonas' frustration and also the reason for his increasing alienation between himself and his old friends. The memories he receives make him wiser and cause him to question far more in his community, meaning he is not able to play and enjoy the life that his friends have in the same way:
His childhood, his friendships, his carefree ense of security - all of these things semed to be slipping away. With his new, heightened feelings, he was overwhelemed by sadness at the way the others had laughed and shouted, playing at war.
It is a combination of these different factors, and of course, the revelation of the true nature of "release" that drives Jonas to make the decision to follow his instincts and pursue justice and fairness by leaving the community with Gabriel to save him from "release".