[greg]

Very interesting topic. A few checks on current comments:

The metaphor of the brain as a carefully portioned soup of chemicals should be retired. The brain is a modularized information processor. While you can pour in more "happy" molecules (try morphine), this still causes a cascade of other effects. Prozac doesn't start working for two weeks -- figure that one out!

Drawing a line between clinical depression and healthy sadness is easier said than done. Almost no one gets depressed without triggering life events. The difference is in depth and recovery. In any case, we still don't have a good idea what is "healthy" about sadness.

It is really hard to judge the long-term benefits of depression. Evolution plays out over generations. Modern society is not what we are adapted for anyway. Maybe we should study siblings of suicides to see if they are somehow more biologically successful on average.