Although some geniuses have mental disorders, don't make the mistake of thinking that all people with mental disorders are geniuses or have special insights - we're just run-of-the-mill people that run the gamut of abilities and personalities. And "happy" is not the right word to use for depression. As Peter Kramer said in his book, AGAINST DEPRESSION, the opposite of depression is not happiness, but resilience. If you suffer from true clinical depression, you can appreciate this distinction.
Agreed. Depression isn't about happiness versus sadness. Depression is about feeling so overwhelmed by everything in life, about every problem and issue being the same "volume" in your head and being unable to prioritize. This leads to a feeling of helplessness and impairment and withdrawal from decision making - a cycle that perpetuates itself.