Answers to the above selected points -
1. Happyness and unhappyness are not real things because they have no material existance. They are simply ways of thinking with the person who suffers from depression having dark-negative thoughts. In psychology they cure depression by teaching a person correct ways of thinking and its called cognitive behavioural therapy (CBT).
2. The problem isnt how intelligent the mind is its the type of perceptions you allow yourself to have. Anybody can ground themselves in dark-negative thoughts and lead themselves into the abyss. People that do havent learnt to manage their own perceptions so they need teaching it with CBT.
3. Why the doubt? Intelligent people dont doubt themselves people who have problems in the way they think doubt themselves. You've identifed your problem as being the way you think without realising it.
4. You dont acquire happy because it has no material existance. If you try to acquire it by gaining wealth, power, love or material possessions then you'll never find it. You need to look inwards not outwards and see its the thoughts you allow yourself to have which make you unhappy. By only allowing yourself to have thoughts which make you feel good then you find happyness.
Sometimes snapping someone out of depression is hard because they are so locked into their dark-negative thoughts they cant break them. If thats you then you need to see your doctor.
I'm not actually depressed myself, and I agree with a lot of what you said. Perception does have a lot to do with whether or not people are able to find joy in their life, and my personal motto for life is that you cant be happy unless you let yourself. This goes for pretty much anything. My line of reasoning before is that "intellegent" people are more likely able to perceive things in a varity or ways, and those perceptions of the world can be negative, and like you mentioned it can be hard to break people out of those dark thoughts.