Final answer:
Greg's chronic symptoms such as prolonged sadness and withdrawal indicate he is likely experiencing major depression, a mental illness that requires at least two weeks of consistent symptoms for diagnosis, but his have persisted for over two years.
Step-by-step explanation:
Greg's long-term friends describe him as being chronically down, pessimistic, hopeless, withdrawn, irritable, unhappy, listless, and inactive for over two years. These symptoms indicate a significant level of distress and impairment in social, occupational, or other important areas of functioning, consistent with a mental illness known as major depression. Major depression is a mood disorder characterized by a prolonged period of sadness, a lack of interest in activities once enjoyed, and a range of emotional and physical problems.
Considering the duration of Greg's symptoms, they satisfy the diagnostic criteria for major depression, which requires that these issues persist for at least two weeks, but in Greg's case, they have lasted much longer. Treatment options for major depression often include anti-depressants and psychotherapy, as well as lifestyle changes like regular exercise, improved sleep habits, and nutritional adjustments.