First, why did he proclaim this message of judgment to the people of Nineveh? This is simple: he was commanded by God to do it. In fact, he was so sure that God was commanding him to do it that the first two chapters of Jonah are an account of Jonah's efforts to disobey God's command. Eventually, he relents, and in chapter 3 travels to Nineveh to address the people.
So what exactly was Jonah expecting when he proclaimed God's message to Nineveh? What was his end game? It seems that he felt he was making a simple proclamation to let the people know that when judgment came, they understood that God was the source of that judgement. No chance for them to be confused as to why they were being destroyed. They had been in rebellion to the one true God, and they were about to reap their reward.
It is clear that this was his intent in the first verse of chapter 4. He is angered that God stays his judgment because of the repentance of the people. The prophet's desire was for God, in all of his holiness and glory, to strike down these evil people who had for so long been against him. There was no thought of any other outcome. Total destruction of God's enemies was at hand, and he had played the part of the mouthpiece of God proclaiming their doom.
What does that say about his understanding of God and hope? Could it be that even a prophet who is close enough to God to receive a direct word for a specific audience doesn't understand who God is and what his ultimate ends are? Did he believe that their was no hope for the people of Nineveh, and that God didn't care about them even has he made the decision to destroy them? Apparently, that is exactly what he believed.
What is amazing is that Jonah thought that his own righteous perspective was the same as God's. While God did care about each of the people of Nineveh, as people created in his own image, even as he was at the point of destroying them for their sin, Jonah did not care about them at all. He wanted to see justice served, and justice in this case was the destruction of these sinners. What's more, he actually becomes angry to find out that God has a soft spot in his heart for these rebellious people. His privileged status as a Jew and as a prophet were not all that privileged when God demonstrates his love (as well as his mercy and grace) to a rebellious people.
His further actions are even more telling: Jonah is much more grieved about the death of his shade tree than he is at the prospect of an entire city destroyed. His own comfort is much more important to him than the eternal destiny of Nineveh. He allowed trivial inconveniences and discomforts to completely cloud his perspective on what is most important in the world, that humanity has a hope through the one true God.
How often in our modern Christian living do we find ourselves acting like Jonah? Like Jonah, all of us (not just those in vocational ministry) have a message from the Lord. We have been called to proclaim the Good News of Jesus Christ wherever we are placed. While I cannot think of any vocational missionaries who would ever have this kind of reaction to the repentance of the people they have been called to work with, I fear that many of us, in our day to day lives living amongst people who we are called to reach with the gospel, have as little care for them Jonah had for Nineveh. We are blinded by our own desires, our own agendas, our own hobbies, or our own comforts. These people are all around us, and yet we do not see them. When we do see them, we see them as enemies, in an "us versus them" kind of way.
Let's be a people who see the lost in our world, and actually care for them. Let us put our own rights, desires, and comforts behind theirs. God asked Jonah at the end of Chapter 4, "You pity the plant... And should not I pity Nineveh, that great city, in which there are more than 120,000 persons who do not know their right hand from their left...?" Sadly, we don't find out how Jonah responds to this. But how will you respond. What if God asks you the same question about your city? "And should I not pity Houston? or Atlanta? or San Sebastián?" Our response: "Yes Lord, and I should too."