Are you sure you see the reality out there? That is the question. The brain does not receive an image like a camera does. The brain is not a passive processor. What you see is constructed in the brain. How do you see with your brain?
What causes your depression: serotonin or what you experience? The answer: the question is invalid! Both play a role, and are interconnected. You cannot say that one of these is the cause.
If you cut your finger the pain appears to originate at the site of the wound. But that is misguided. Pain is an emotion that is generated in the brain itself. Pain is always in your head.
A lot of people think antidepressants are worthless. Is psychotherapy better than an antidepressant? There is no evidence for this. It seems to me that antidepressants suffer a disadvantage in our perception as compared to psychotherapy, in that their mechanism of action is quite invisible and not instinctively comprehensible.