Jonah's different responses to his and Nineveh's deliverances clue us in to what the great message of this book is, because he is not just the prophet delivering God's message, he is also the one to whom God is delivering His message to.
The humanism of our culture is burning out and idle utopian delusions occupy those waiting for the end. While humanism is committing suicide, this is a time of glorious opportunity to bring the saving Gospel of Jesus.
Jonah's self-pity so warped his moral sense that he had more compassion for a plant than for a city full of people. Our zeal for the truth, justice, and service of God must always be undergirded by love for and submission to the person of God Himself.