It is true that if your feeling of sadness is associated with a stressful event in your life, you could get out of it soon enough. However, if your depression is ongoing for six months or more, then your brain chemistry may have changed as a result of the prolonged sadness. You could be clinically depressed. Depression can strike at any age, including in childhood. However, the common time of onset is early middle age, and depression is particularly rife among the elderly, as a reaction to the facts of growing older - the death of a spouse or friends, the physical limitations of age and the impending confrontation with death.
Elderly widowers are particularly susceptible to suicide.
Depression is one of the most common psychiatric disorders. Because it is so common, many people feel that if they ignore the depression, it will disappear. This may be the clue that your illness has progressed to the stage that it needs to be treated. If left untreated, depression may lead to suicidal ideation or attempt. Suicide is a real threat in depression.