I think the focus on brain chemistry for depression is as silly as for any other emotional state. For instance, you don't need to know about the chemical makeup of "angry", just that you're angry, and then look at what caused it.
If a depressed person genuinely thinks their life is good and that they have everything going for them, this is often from lack of realisation or self deception.
To pick an easy example, someone who's got a successful well paid big city career being depressed, because internally they don't want that and just did it because others told them that's the path to happiness, and they've repressed their subconscious desire to say raise a family in the countryside instead.
If a depressed person genuinely thinks their life is good and that they have everything going for them, this is often from lack of realisation or self deception.
To pick an easy example, someone who's got a successful well paid big city career being depressed, because internally they don't want that and just did it because others told them that's the path to happiness, and they've repressed their subconscious desire to say raise a family in the countryside instead.