DEPRESSION – WHEN IS SADNESS A PROBLEM?
According to Wikipedia , sadness is an emotional pain associated with or characterized by, feelings of disadvantage, loss, despair, grief, helplessness, disappointment, and sorrow. Sadness is one of the so-called basic emotions.
The most frequently reported antecedents of sadness, from high to low prevalence, are: problems with friends, death of friends, sickness (own or others), death of relatives, permanent separation from friends, problems with relatives, failure in achievement situations, bad news (social context), bad news (mass media), temporary separation from friends, solitude, end of the pleasurable experience and general depression.
According to the American Psychiatric Association , Depression (major depressive disorder) is a common and serious medical illness that negatively affects how you feel, the way you think, and how you act. Depression causes feelings of sadness and/or a loss of interest in activities you once enjoyed. It can lead to a variety of emotional and physical problems and can decrease your ability to function at work and at home.
Depression symptoms can vary from mild to severe and can include: Feeling sad or having a depressed mood, loss of interest or pleasure in activities once enjoyed, changes in appetite — weight loss or gain unrelated to dieting, trouble sleeping or sleeping too much, loss of energy or increased fatigue, slowed movements or speech (these actions must be severe enough to be observable by others) or increase in purposeless physical activity (e.g., inability to sit still, pacing, hand-wringing), feeling worthless or guilty, difficulty thinking, concentrating or making decisions and thoughts of death or suicide. 
When does sadness become a problem?
Historically, sadness has been considered to be one of six ‘basic’ emotion facial expressions, along with happiness, anger, surprise, fear, and disgust.