Intense and long-lasting sadness, or in more modern terms—"depression"—has been with us since the first person walked the earth.
Most depressions are caused by resisting situations and circumstances a person does not want. The unwanted situation is fought against, and if it does not give-in, depression sets in.
Some depressions are caused by biological imbalances (medicine side effects, hormonal imbalances, heart-attack recovery, etc.). In my clinical practice, "biological depression" is rare, whereas "circumstantial depression" is the most common. If you are "depressed," chances are it is circumstantial. (If you are not sure, get an evaluation by a competent mental health professional.)