Clinical depression is different from being sad about things. The underlying cause of clinical depression can be an imbalance in your blood chemistry.
for instance I went to a Psych doctor and was told I had real good reason to be depressed “your life really does suck lemons” he said.
/sarc
Depression is diagnosed by behavior, so by definition, it is not a disorder of altered blood/brain chemistry (we diagnose problem of chemistry by measuring chemistry and diagnose problems of behavior by measuring behavior). Further, there is no scientific evidence that “imbalance in blood chemistry” can be the underlying cause of depression.” Nearly all of the studies on depression take observations of overt behavior (both publicly observable nonverbal behavior and verbal self-reports of feelings, thoughts, etc.) and infer an underlying chemical/neuro-chemical state. The studies that actually measure blood and brain chemistry are done with people who have already been identified as having clinical depression. See the problem here?
There is little doubt that people with clinical depression have altered blood/brain chemistry but to date, no one has shown that those cause depression; rather, people have shown that changes in the environment produce clinical depression and the co-occurring changes in chemistry (which is likely why behavioral interventions are, time and again, found to be the most effective interventions).