Diagnosable Disorders and Suicide
"It is tempting when looking at the life of anyone who has committed suicide to read into the decision to die a vastly complex web of reasons; and, of course, such complexity is warranted. No one illness or event causes suicide; and certainly no one knows all, or perhaps even most, of the motivations behind the killing of the self. But psychopathology is almost always there, and its deadliness is fierce. Love, success, and friendship are not always enough to counter the pain and destructiveness of severe mental illness "
— Kay Redfield Jamison (Night Falls Fast: Understanding Suicide)
There are many diagnosable disorders that are related to suicide. Here you can read about those disorders, everything from Mood Disorders (which includes Seasonal Affective Disorder, often referred to as SAD) to Adjustment Disorders, and the research related to suicide.