I postulated somewhere a couple weeks ago that anger is the psyches social immune system. It keeps out what is unhealthy naturally and spontaneously. It works much faster than our brains, even if it sometimes makes mistakes (which I suppose makes chronic anger a sort of social autoimmune disorder, in that it attacks what might otherwise be healthy).
Today I would like to assert that sadness means something is lacking.
It is not unreasonable to assert that our natural state is happiness and engagement; that people who are truly happy and truly fully engaged seem rare in our own society–at least as I see it, and perhaps I myself am being stupid, which is a strong possibility–does not mean that if we get what we need emotionally, that health and happiness is not a spontaneous and quick result.