He really didn't want to go to Nineveh and waste his time warning them that the Lord had noticed how sinful they were. To avoid the task, he jumped on a ship. That plan did not work for Jonah, and he finally decided to obey the Lord.
He preached to them. They listened. They believed. They repented. God decided not to destroy them.
Jonah's response was shocked anger. "I knew this would happen. This is the way you have always been. You talk about sin and punishment, but what you do is forgive. Why did I have to go to all this trouble, this trip, this preaching? I give up."
He sat down under a bush, waiting to see what God really was going to do.
We can find ourselves in this story. We can see times that we have been Nineveh. Times that we can say we didn't know better and times when someone had instructed us forcefully enough that we lost the defense of ignorance. Times when we did repent for our past doings.
We can see times that we have been Jonah. God wanted us to do something, and we really didn't want to bother. Times when we have done what we thought God wanted and then we weren't satisfied with the results.
How hard is it for us to accept that someone else's sins can be forgiven?