Psychomotor agitation or retardation nearly every day observable by others. Fatigue or loss of energy nearly every day. Feelings of worthlessness or inappropriate guilt. Diminished ability to think or concentrate, or indecisiveness, nearly every day, as indicated by either subjective report OR observation by others. Recurrent thoughts of death, suicidal ideation without a specific plan, or specific plan for or attempt at committing suicide.
Nobody pretends to fully understand the causes of depression.
Much research over the past several decades points to the neurochemical factors as being MORE important than psychodynamic ones. There are other people who lapse into depression seemingly at the slightest life stress. What's more, once the depression is begun in these people it tends to self-perpetuate, like a vicious cycle, long after the precipitating life event has passed. In general, antidepressant medications are far more effective in a far higher percentage of cases. Most clinicians feel that a combination of both psychotherapy and medications is best - that the medications are often a prerequisite for making psychotherapeutic progress, while the psychotherapy in the long run is what allows you to eventually be weaned from medications.