When God saw what they had done and how they had put a stop to their evil ways, he changed his mind and did not carry out the destruction he had threatened....Then the Lord said, “You feel sorry about the plant, though you did nothing to put it there. It came quickly and died quickly. But Nineveh has more than 120,000 people living in spiritual darkness, not to mention all the animals. Shouldn’t I feel sorry for such a great city?”
Because God changed His mind and granted mercy to the people of Ninevah (whom He had sent Jonah to inform He was going to destroy because of their wickedness), Jonah got mad at God. Isn't this just like us? We want justice, and when repentance is real we still want "justice." We can accept an apology as long as the consequences are still appropriate, but when we're called to accept the apology and see blessing in place of discipline we get upset and, like Jonah, pout. Maybe it's that you know the life's history of someone who seems to be blessed now and you don't understand. Maybe the consequences of someone's sin didn't end up like you thought they would (or should?). Who knows why we pout when God doesn't do what we think He should...but shouldn't we suck in our lower lips and instead be thankful that God doesn't deal with us as we deserve, either?