I think you're overstating the success of chemical treatment. I'm not extremely well versed in the subject, but I've read a number of books and articles that seriously call into question that idea.

It would seem that "talking about it" is actually more effective than medication in many circumstances (e.g. the research on cognitive behavioral therapy versus medication).

The consensus at this point seems to be that we do not have any real causal theory for most mental illnesses, including depression.