The Assyrians were powerful and brutal. Jonah disobeyed God’s command to go and preach to them because he was afraid of what they might do to him. When he eventually went and the Assyrians repented, Jonah was not happy because they were spared judgment. 

Jonah fell into the trap of being more concerned about himself than about others. His very words—“It is better for me”—indicate that his priority was on his own desires rather than the well-being of others—even others who had been his enemies in the past. Instead of rejoicing at the Assyrians’ repentance, Jonah was mad at God for having mercy on them.

The apostle Paul wrote that we should look not only to our own interests but also to the interests of others (Philippians 2:3-5)—the very attitude that Jesus Himself demonstrated. 

Forgiveness saves the expense of anger, the cost of hatred, and the waste of spirit. 
Hannah More