What is the point of being a prophet?
The woman at the well said, "Sir, I see that You are a prophet..." and then she told everyone else, "Come, see a man that told me all the things that I've done..."
...and Jonah is an example of a prophet. And, in his case, he was reluctant to tell the people of Nineveh what God said, and scripture tells us why he was reluctant. He didn't want God to save Nineveh. Jonah would've rather it had been destroyed, but Jonah must've known, or at least have been pretty confident, that the people of Nineveh would believe him and repent and so he attempted to avoid the mission.