people who suffer from depression because of environmental factors
Folks who have brain wonkiness can't fix it by fixing other life problems. But people who are depressed because their life sucks absolutely can see their mood improve by fixing other problems.
This is not at all what the field of psychology has identified as the "general sources"
I will share that it is both more complex than many believe - and also less mysterious. "it's chemical" gets overplayed, while our relationship with our emotions is often left unexamined. There's a great deal of interconnectedness and sometimes cause & effect become confused or are tightly coupled. In the end, we're still learning, and everyone is different.
(edited)
Presumably, it's always a biochemical issue, though sometimes the biochemical state may be triggered by events in the social context.