The New Scientist asks Is it really bad to be sad. Apparently there are some compelling security arguments for sadness:

Hard evidence for the importance of sadness in humans is difficult to come by, but there are lots of ideas about why our propensity to feel sad might have evolved. It may be a self-protection strategy, as it seems to be among other primates that show signs of sadness. An ape that doesn’t obviously slink off after it loses status may be seen as continuing to challenge the dominant ape – and that could be fatal.

This does not really surprise me. I like the old Hungarian saying “the believer is happy, the doubter is wise” because it emphasizes that doubt may in fact have a silver lining. This is usually forgotten in a “yes” or “just do it” culture. So the implication here is if you use your capability to be sad, you actually may be smarter and safer.