|Grievance = 100 - Tax Rate - Happiness|
Normally the Grievance value is 0. Unless something happened to your city and army.
What affect GrievanceEdit
The following factors affect Grievance as well:
Relive Famine: It consumes 13500 Grain and it will +5 Happiness and -15 Grievance;
Pray for Blessings: It consumes 13500 Gold, +20 Happiness, -5 Grievance.
If your city get attacked by the other players and they win, Grievance will increase.
Your Grievance will increase when you get plundered or conquered. But if you have been plundered for 5 times, your Grievance will not grow with any more plundering.