I didn't make any references to therapy.
Therapy works for some [people and circumstances] and not for others.

Yeah I changed it to lifestyle changes a second ago. How often do you think mental disorders can be cured or at least aided without the use of drugs? I think drugs are needed in alot of cases.

How many do I think can be at the very least improved without the use of drugs? Most, if not all of them, but I'm not really qualified to say.

A significant improvement though? Yes, positive lifestyle chanes can definitely help, but sometimes drugs are simply needed to help along the process. It's the way you balance drugs and lifestyle changes and/or therapy to see a maximum benefit. Just considering depression and GAD, many people just don't have the proper serotonin levels to be able to cope, so SSRIs enable them to achieve the maximum benefit.

Why do people with depression and anxiety not have proper Serotonin levels? Are the lower serotonin levels a manifestation of the respective disorder, or vice versa?

I have no idea. That's a good question though. All I know is that there's a strong correlation between the two.

Well, it's a very important question to answer. When someone is trapped in a burning building we don't have a tailor outfit him with an asbestos suit and leave him inside the structure fire, but this is simply an apt metaphor for using SSRIs or MAOIs to treat depression. Unless the depression itself is congenital, there must be a root cause for the imbalance. Address the root cause of of the imbalance and you actually productively address the disorder, not just the symptom(s) of the disorder.

If it were the case that the disorder itself caused the unbalanced serotonin levels, who's to say that the root cause is something that can be treated with lifestyle changes anyways, and not something that can be treated by some other chemical means? These are hypotheticals though. It would be best to get the answer to that question.

That's a pretty fucking big "unless" friend.

Why?