The DSM5, the most recent version of psychiatry’s diagnostic bible, makes it possible to classify grieving that endures beyond a rather brief span of time as a mental illness.
This pathologizing of grief has ancient roots extending back at least as far as the Stoics, whose stern ascetic morality preached a perfect indifference that eschewed all passionate attachments. The ideal of selfless asceticism was carried forth in early Christianity, showing up dramatically, for example, in the Confessions of the prominent 12th century monk, Saint Bernard, who was wracked with guilt over his grief for his beloved dead brother. His brother, after all, was enjoying eternal happiness in heaven, so Bernard could only feel his grieving his loss as a manifestation of a wicked selfishness on his own part.
The pathologizing of grief was continued by the philosopher Rene Descartes, usually considered to be the initiator of the Enlightenment and Modernity. In letters to Princess Elizabeth of Bohemia and Constantijn Huygens, he warned that sadness and grief could cause serious physical illnesses, and he recommended a form of mental discipline—reminiscent of both the Stoics and contemporary cognitive-behavior therapies—in which the imagination was to be directed away from the sources of emotional pain and toward objects that could furnish contentment and joy. In the current psychiatric and medical climate pathologizing grief, psychiatrists (and even general practitioners!) are conflating painful feelings with clinical syndromes and prescribing anti-depressant medication for naturally occurring intense or prolonged sadness and grief.