Even if a scientist can legitimately prove depression is different from sadness, you will not believe it, because it requires a degree of subjectivity you will not tolerate. You expect a kind of purity, an objective, emotionless vocabulary with it's associated numeric data; data that has been wiped clean of the people and the lives of those people it describes, to describe that which is fundamentally indescribable to someone who has never experienced it.

Sometimes compassion means saying 'okay, I understand, that sucks?'.