Presonally, I think it can come from a variety of factors. And I've seen that in my work. For some, it could be something biological. That element of misfiring neurotransmitters and the brain. Others, it's a mixture of neurological and environmental factors, genetics...I guess it depends on who you talk to.
The different types of antidepressants are meant to increase/block our neurotransmitters. Specifically the ones that can influence depression. I don't think depression medications can ever cure by themselves, and shouldn't be used as such. I suppose they are simply to 'take the edge off'.