Inability to control moods, resulting in pathological mood chronification.

From the biological perpective the duration of depression is depending on what medication you get and how long it takes before it works properly e.g. 2 months. It also depends if it is a reactive or endogene depression.

In primary care, depression is often connected to and caused by bad personal, family, work, or economic circumstances. People with depression can learn how to deal with these situations, either by improving their depressive symptoms as a result of treatment or by developing effective coping strategies. But in real clinical practice, we can see that depression is very often chronic because the "external" circumstances that cause depression continue and the patient can't face them successfully.