Customer happiness is represented by the hearts shown over their heads while in line, or on their table when seated. As customer parties get angrier, they will lose hearts. When a customer party has lost all of its hearts, the party will leave. 

When a customer party has only one heart left, the party will begin flashing. When a customer party is on the verge of leaving the Diner, their eyes will be set aflame! Avoid angry customers by using upgrades like the jukebox, coffee machine, or menus from the podium.