This is the $64,000 question. Either we're all acutely aware of ourselves and we're just feeling the normal human condition multiplied through that awareness, or we really aren't functioning correctly. I've yet to work that out myself.

All I do know is when I do things that literature has said will reduce symptoms of well known and well documented behavioural issues, my own go away a little bit.

As far as I understand - no. However it's not like other people don't get sad, and always stay happy and motivated, they are just able to break out of their bad moods quickly enough.

Depression is very widespread though. Up to one third of population will suffer from major depressive episode atvleast once in their lifetime(Major depression is the most serious subtype of depression). And around 10%-15% of population currently suffer from various types of depression.