Panda has 5 attributes:Joy,Beauty,Training,Health and Happiness.The players need to keep your Panda's joy,beauty,training and health at a high level so the happiness of the Giant Panta would increase.You can click on each icon of attribute to check items can by applied on each attribute.As the Pand happiness is assessed by all 4 attributes , so just keeping 1 or 2 attributes at high level can not substantially increase in the Panda's happiness.So you need to take care of all 4 attributes to make sure them at a high level