Okay I will answer to the best of my ability. I notice you are making me seem like denying the reality and intensity of the pain that people with depression and anxiety disorders suffer from.... No, I'm not denying the reality of it, I am affirming it. I believe there is no pain worse than that faced by people who are depressed and suffer from anxiety disorders. People who are depressed have it the worst. And on the other hand, in a way you are right. I say that people with depression and anxiety are losers, not in the sense that it's their fault for their condition, but in the sense that they are not getting the most out of life while they are afflicted by the said condition. And instead of telling them that they are condemned to a life of living with the said condition, a life of misery and fear, I tell them that there is hope and that there must be a way not only to make their condition manageable, but to cure it! I say that there is hope! Now to offer a few quotes:

"Depression can seem worse than terminal cancer, because most cancer patients feel loved and they have hope and self-esteem" - David D. Burns

"A human being can survive almost anything, as long as she sees the end in sight. But depression is so insidious, and it compounds daily, that it's impossible to ever see the end" -Elizabeth Wurtzel

Now I will answer your points more specifically:
1. You say you are a man of science. But the fact of the matter is that science itself knows very little about the human condition. I have spoken with doctors, and medical professionals who have studied for years to earn their degrees and have seen hundreds of patients. I have done my best to investigate this condition of depression and anxiety. And in the process I found several interesting things about so called science. Modern science has become a sort of religion, in the sense that it is dogmatic. It refuses to go out of the tried methods and experience with new things. I have spoken with doctors who have worked in hospitals with conventional medicine and have also worked with alternative medicine. And I see a resounding conclusion from them... alternative medicine, while it can't be tested according to the double trial principles with which normal pills are tested DOES cure patients. This is not what I believe, this is what doctors have told me. They have seen it work. Now I believe that I can rely on the honest opinion of men of medicine who I have spoken with. They know better than you or I know. You say that fear has played an important evolutionary role. However, I dare to disagree and I will explain why. And you cannot claim that it's scientific that fear has played an evolutionary role and therefore I must be wrong. I dare say that it's not fear that has played an evolutionary role, but rather consciousness. It is because you are conscious of pain that you don't put your hand in the fire. It's not because you FEAR pain, you simply don't like feeling pain. Suppose you had no fear of putting your hand in the fire. Why would you put it? Do you want to feel pain and hurt your body? If that's what you really want to do, then I don't think fear can stop you. If I don't live with fear that doesn't mean that bodily harm is irrelevant. How does that even follow? If I live without fear all that means is that I am completely free to go out there and do exactly what I want to do and what makes me happy! If putting my hand in the fire makes me happy, then fear or no fear, I will do it. It's not fear stopping me, it's my consciousness. The fact that I'm aware of the consequences of my actions and I don't want those consequences to happen because I prefer some other consequences to happen. I don't want to have a burned hand because I won't be able to play the piano which I enjoy after that for example. Do you really think that you don't put your hand in the fire because you fear it? I don't think so. I think you don't put your hand in the fire because you wouldn't enjoy the feeling of it or the consequences that follow, simple as that. Now another argument... children, if you don't stop them, will put their hand in the fire out of curiosity. So it seems your supposed evolutionary mechanism of fear is not doing its job to prevent children from putting their finger in the fire.

2. I have never said depression is CAUSED by lack of desire, I said that depression has lack of desire as a symptom, and lack of desires maintains it by maintaining a low self-esteem in the individual. I agree that we are all individuals and something will not work for all people, however, if a thing works for a person, it will likely work for another one as well. This is a central principle of medicine. If 10 people are cured of X by taking Y, then most likely the 11th person will also be cured of X if he takes Y. Not necessarily of course, but most likely. As for the chemicals in the brain, yes I agree with you. Depression is an imbalance of chemicals in the brain. The only question is what causes the imbalance? The answer is that nobody knows. Personally I believe that the imbalance is somehow caused by us and our own mental activity. And therefore we have the power to change it. And this is not cold and grim as you say, but hopeful. This belief gives me and others who have it the hope that we can overcome depression and anxiety and we don't have to live our lives haunted by these conditions. EDIT: Also since I'm on this topic now... I must confess that this belief is not only beneficial to hold, it is also reasonable. What is unreasonable is to think that there's a chemical imbalance in your brain for no reason. What reason could this imbalance have? Clearly if you are eating the right diet, and taking in the right amount of the things your body needs then the only reason that could be is psychological. Something in your psyche causes your brain to produce a chemical imbalance. And since you enjoy science, you should be aware that nothing within our biology implies that we should be conscious, or aware of what is happening. In theory, one could make a robot that would behave exactly like a human being. Yet one thing the robot will lack is awareness. And you can't prove awareness or consciousness. You can prove it for yourself sure, but you cannot prove that I or anyone else is conscious. From your point of view I could as well be a robot, doing all this without being aware that I am actually doing it. So this consciousness of ours seems to have important effects on the psyche AND on the chemical balance of the brain. Look at the studies that have been done on people who meditate frequently and you will see. A strictly materialistic point of view is unreasonable in my opinion given the current understanding of science.