So, people who've led relatively normal, successful lives with good relationships and have depression might be experiencing it so deeply because they haven't had much exposure to sadness? Sounds like a reasonable theory to me.

That would also explain how people are able to get out of unexplainable depression after a period of time, and why people sometimes seem to seek out more sadness when they're already sad. They're looking for perspective.

Awareness is the goal of some forms of meditation. I'd like to see more studies on the effects of meditation on those who have various forms of depression.