In the human brain this is an area which actively develops both prenatally and during the first two years after birth. This finding is particularly relevant for mental-health professionals because the hippocampus is an area that also plays a part in controlling the HPA axis – a system that operates in stress responses.
So it has helped understanding of depression as for as is the case of new borns.

On the other note how i experience depression is due to memory , i play the event time and again in my memory which depresses me emotionally and keep on thinking about what if I could have done "specific task/situation" in this way or that way.
In second types of "events" which depress me are "revenge related" that i keep the memory of that event alive and want to avenge for that and if avenging is out of my reach then "every other scenario" relevant to previous experience "increases" my "stock" of depression. which was depicted in your article as:

Prior to his treatment, Mr. B thought he was a failure at everything.

But when it gets shared and you feel other people also go through such states in similar scenarios then one feels peaceful.
Similarly if a person keeps in mind that a "specific event" was done intentionally to "stay in one's memory and haunt him", then depression goes away fast as compared to not focusing on the patterns of depressions previously experienced.

Very nice read but was a bit difficult and long as well :-)