As I said earlier, I believe there is often an essential link between unthankfulness and depression. It seems to me to go like this... we grumble and complain about our circumstances rather than be thankful for them as we should (ultimately) as we are exhorted in Romans 8:28-29; our unthankful disposition leads to anger, usually suppressed in some manner and to some degree; this anger eventually produces depression.
I'm not trying to suggest that this simplistic progression is trivial to overcome. On the contrary, I see it as insidious, pervasive, and akin to the "strongholds" mentioned in 2Cor. 10:4. I'd be pleased to hear anyone's perspective on these thoughts...thanks!