You say "our brains do work very much on chemicals" and then reject the idea that chemical imbalance can be the cause of mental illness? This makes no sense.
Mostly I find the term "chemical imbalance" to be somewhat clumsy. It doesn't imply that in itself it is a cause, but rather that there are parts of the brain that are either not producing enough neurotransmitters or the receptors are too closed off to them. Why are those parts of the brain not functioning? What is an actual "balance" of chemicals? And how might your lifestyle choices affect those balances?
These are good questions that point to the actual root causes, not wishy washy terminology.
Let me put it this way. Lets say you have an iron deficiency. Is that necessarily the cause of some intestinal pain that you might be having? Or could it be a larger cause like celiac disease? A deficiency alone does not necessarily constitute a cause.
Let me know when you become a doctor and I'll listen to you. Until then I'll listen to what actual doctors have told me. Depression is caused by chemical imbalances in the brain, chemical imbalances that often times can only be corrected with medication.
Spare me the whole doctor spiel. You only say that because you disagree. If I were to use this reasoning, I shouldn't listen at all to what you have to say on the matter either. And it wouldn't really be too hard for me to pull up examples of doctors who believe that "chemical imbalances" aren't the sole cause of depression.
Take some time to read my response to tsq above. I have no qualms with people taking anti-depressants, and yes chemical imbalances are a part of the processes that affect mental illness, but I still do not believe that it can be described as a cause so much as an effect or symptom.