From Alan Jacob's The Narnian:
Transformation is not optional but mandatory for Christians. This was Lewis' consistent position. After all, he had undergone his own transformation, discovering "depth under depth of self-love and self-admiration" and submitting to the life-long discipline of being purged of such sin. We must die in order to live, hate our lives in order to find them, give up what we think of as ourselves in order to gain our true selves. And this is the most difficult of tasks.