Depression can take various forms.
It may be triggered by a period of severe stress or a distressing life event, such as a breakdown of a relationship, a loss of a loved one, feeling trapped in a situation, or any other traumatic event.
Very often a time of depression can be a revisiting of an earlier time of depression, even from childhood, when it may not have been recognised. Something in the present may trigger the depression, and it is as if the mind is trying to resolve it this time round.
It is often a sign that the self-structure, that been developing since one's earliest days and has been coping over the years is struggling, and cannot cope any more without changes. One's beliefs about oneself and one's healthy defences may be compromised. The whole body/mind system becomes affected.
Depression can be taken as a sign that something needs to change, which, if heeded can be a starting-off point for a much stronger and healthier sense of self. Good psychotherapy can be a great help in the search for such restoration, and even lead to a much better sense of self than was experienced before the depression. Treated in this way, depression is much less likely to re-occur.