Yes, they are based on chemicals. Also, did you know that by smiling and being nice to people you can add chemicals into your body that make you happy?
And yes, chemicals affect everybody differently. But it's scientifically proven that you have control over your feelings. Unless you have some incredible toxin overload, or you have a glandular problem, you can do something about depression without a doctor.
Unless you think that people who have chemical addictions like smoking and drinking also don't have control over themselves in a similar way?
(edit: @baddox - https://www.quora.com/Can-anyone-really-quit-smoking )
Edit: > Would you say when your eyes become bad just try harder and work on yourself,
I learned from a Swiss airforce fighter jet pilot about eye exercises to help with vision loss. (maybe it's common knowledge, but that's where I heard it) So I guess it depends.