Depression is definately not something to be thought of lightly - especially since the people suffering with it can't take jokes very easily.
The causes of depression vary widely. Although in some cases the cause is obvious, such as the death of a close friend or family member or a severe injury, the cause is often unknown. Much progress has been made in this field, however. For example, just in the past decade or so, a lot has been found out about the neurotransmitters whose abnormal activity levels (some, such as Serotonin, are extremely low when contributing to depression, others, such as CRF, are very high in depressed people). And it has also been proven that depression is a recurring disorder; it never really goes away, just hides out for a while. 30-50% of all people who become depressed at one time in their lives can be expected to have a recurrent episode, and each time another episode occurs the probability of having another one goes up. Also, thanks to recent biochemical research a number of other causes have been identified. For example, long periods of extreme stress deplete several neurotransmitters in the brain and give some others a boost that are known to contribute directly to depression. Also, genetic links to depression have been determined, and a number of links to childhood conditions have been made. For example, growing up in an abusive home can contribute by forcing one's brain to lower Serotonin production (Serotonin calms you down and makes you generally less alert; not a good idea if you are always wondering where your next beating is coming from), and over long periods of time while the brain is still developing, the brain will lower its production of Serotonin permanently, which contributes to depression in later year.
The symptoms of depression are also varied, and in many people take opposite extremes. Many people suffering from depression will either develop insomnia or a constant need to sleep, refuse to eat at all or always be stuffing their face, act on impulse or insist on thinking everything through twelve bazillion times for every last detail (the latter, if nothing else, puts them at much lower risk for becoming suicidal than the overly-impulsive incarnation of depression). Depressed people also tend to be very aggressive, extremely jumpy, and very easily irritated. Associated emotions include guilt, inescapable/unexplainable sadness, and inferiority.
An example story (names and details have been obscured so that this person can not be recognized): Mary Williams was a fifty-six-year old married woman who was employed as a secretary. She went to see her family doctor complaining of insomnia, poor appetite, and weight loss. Her symptoms had begun approximately three months earlier when she fell and fractured her wrist. The injury was not serious, but she had to have her hand in a cast and was unable to work for several months. She found this rather distressing, since she had been employed as an executive secretary at IBM for nearly fifteen years and was convinced that her employer would have trouble getting along without her. She also did not know what to do wither herself during the long days at home. Shortly after the injury, she began to have trouble sleeping. She would awaken at 3 or 4 in the morning and would lie awake for the next four or five hours thinking about things that she had done wrong in the past. In spite of everyone's comforting and understanding responses when she announced her "sins" (which included such things as occasionally using a pen from her office for personal use, forgetting to call her daughter once, and having hit a squirrel while driving) Her appetite diminished and she lost approximately ten pounds over two and a half months. She also began to lose interest in all her favorite recreational activities. When her daughter told her that she was expecting a child in seven months, Mary (who normally doted on her grandkids) responded by shaking her head dissapprovingly and saying that it was wrong to bring any more children into the world, since it was such an evil place. When talking to a psychiatrist, she described herself as feeling extremely sad, anxious, pessimistic, and overwhelmed with guilt, worthlessness and hopelessness. She could not sit still - constantly twisting her hands and playing nervously with her hair. She stated that her family would be better off without her and that she had considered taking her life by hanging herself, and that she would go to hell where she would experience eternal torment but even that would not be punishment enough for her "sins" (mentioned above). Also, ten years prior to this episode she had had an experience that her family described as "almost identical." (Andreasen)