Sadness and Depression

Mood 
Mood is the persistent state of emotion. It is something personally felt by the ‘feeler’ and usually is observable to others too. Some are positive mood states: happiness, excitement, exhilaration, exaltation, while others are characteristically negative ones like sadness, disappointment, gloom, frustration, despondency or depression; to name a few. Everyone goes through different emotions through their lifetime, over the years, months, weeks, sometimes even several times a day. A low mood is a problem and if there was an option, one rather not have to deal with it; yet it is a slice of life that one must devour. Life cannot be positive all the time. However, when a negative emotional state is persistent, and it causes longstanding trouble to the person feeling it, it qualifies as a disorder. 
The truth about sadness 

Sadness is such a common emotion. It's a ‘customary’ reaction to loss, failure, rejection; all of which are inevitable in everyone’s everyday routine existence. All such situations involve pain and the blues they cause are warranted. But staying sad for too long, letting the sorrow engulf you, and inability to snap out of it is not justifiable. That’s the difference between sadness as a passing emotion; and depression as a disorder that needs attention.
The truth about depression

Depression is not simply a state of ‘mood’. It becomes a part of life of the depressed person. Slowed speed of thought, inability to make brisk decisions, not wanting to talk to people; these are the initial presenting features of pathological depression (making it different from ‘just a bad mood’). Gradually the body starts getting affected. Inexplicable loss in appetite, incomplete and unrested sleep with inability to experience pleasure soon sets in. Thoughts become irreversibly negative: life seems unworthy of living, self esteem gets depleted and at times one may believe this is a worthy self-punishment since self regard has been negated entirely.
The progression

Decrease in positive well being is the first sign of depression. Individuals with depression feel sad almost throughout the day, everyday. There is a decrease interest in all activities that previously were pleasurable. They perceive a loss of energy, and fatigue, with unreasonable guilt and self-reproach. Their ability to think and concentrate also goes down since dullness sets in and contributes to the indecisiveness and irritability. Suicidal thoughts may set in in extreme and severe depressed states. People may give up jobs, friendships, and marriages; in several cases their life too. The end point of depression is the lowest low. 

Masquerades and many faces

Depression may mask itself and present contrarily to what is expected of it. Sometimes one may experience ‘atypical’ symptoms like overeating, excessive sleeping, anger (instead of sadness), restlessness, agitation; all of which never arouse the suspicion of ‘depression’. Mood changes during menstruation, chocolate craving, unexplained mood swings, seasonal mood switches are all masks that depression garbs itself in. One must be vigilant. It may occur in bouts for short periods of time (recurrent brief depression), may be seen monthly in females prior to the menses (pre-menstrual dysphoria), may follow a seasonal pattern of coming up in the summer or winter months repeatedly (seasonal depression), or may be continuous and last for years together (chronic depression). The pattern must be observed in order to offer the appropriate treatment.
It’s awful but there is hope

Measurements of social behavior and subjective feelings have shown that depression is one of the most disabling and distressing medical illnesses, causing significant disability. The continuous mental pain seriously influences the quality of life and the person rightly feels that life is really not worth living. The risk for suicide is also the highest in depression compared to all other disorders. Medication and psychotherapeutic techniques help individuals perceive the beauty of life; to ensure that it becomes socially, emotionally and economically more meaningful