It is difficult to understand why the emotion joy causes disharmony since joy is a state of contentment and is the most desirable emotion of every human being. Yet in traditional Chinese medicine joy is not referred to as a state of harmony but rather as a state of agitation and excessive excitement. It manifests with uncontrollable laughter and overall inappropriate behavior.
Joy relates to the Heart. Excessive joy may overstimulate the Heart causing heart palpitations, scattered mind and inability to focus. In time joy may develop internal heat, leading to Heart Fire, which manifests in restlessness and insomnia. In TCM the Heart houses the mind therefore in severe cases excessive joy may manifest in mental disorders. To learn more about the Heart from the perspective of TCM visit the Physiology chapter.
On mental level excessive joy may be restrained by practicing self-control.