I would think it is designed to make build empathy and love for those around us, fostering relationships to ensure the community aspect of the species. Feeling sadness over the loss of a child/friend/etc ensures the desire to protect from harm, and to avoid situations that would purposefully bring on sadness. Even animals feel sadness - you can see it when animals lose their mate. It builds those relationships and encourages people to protect each other.

My theory...

Feeling sad about something will make you try to avoid that something in the future. You'll feel sad when someone close to you dies or gets hurt, so you'll do what you can to help them avoid it. So my guess is that it's an emotion that helps us help others (who are close to us to) survive.

I found this article in Scientific American thought-provoking - that depression, not to mention garden-variety sadness, could in fact be beneficial in evolutionary terms. What do you all think?

http://www.scientificamerican.com/article.cfm?id=depressions-evolutionary

A recent blog post about this by Robin Hanson of Overcoming Bias says the following:

1. Positive feelings predict risk-taking, alcohol and drug consumption, binge eating, and the neglect of threats.

2. Young people who report high life satisfaction earn less money later in life than their less merry peers (think about popular high school students who become losers).

3. Sadness leads to more systematic thinking, attention to detail, and external orientation.

4. Happiness leads to snap judgments and thus greater proneness to racial or sexual stereotyping.

5. Cheerful people are easier to deceive, worse at detecting lies and fingering thieves; they are more selfish, and worse at defending their opinions.

The link is here:

http://www.overcomingbias.com/2012/04/happy-is-far-2.html

Sadness, like all emotions is a signal. It is attempting to let you know something about what is happening.

Usually, it helps you know when something has been lost, like a corollary to praising something (ie if you can praise it, you'll be sad when it's gone). Or it may help you realize retrospectively that something was important to you, if you did not take the time to appreciate it before it was gone.

If you think of a child that starts to cry, the ideal healthy parent response is to reach out and hug that child, to comfort. In that way, some of the symptoms of sadness can help with social bonding, and empathy for those we consider part of our tribe.

Some people are afraid of feeling their feelings for fear that their feelings will rule them . . . but the thing is, whether you allow yourself to feel and be with your feelings or not, they will come out. If you try to push your feelings down and away they'll leak out in unusual ways. Imho, it is better to just honor that they are there and make space for them.

Please read Emotions Revealed by Paul Ekman. One of the emotions he covers is anguish. It is one of the emotions that is tied to the face. I call these the social emotions. Anguish is intended to elicit response from others. I believe it's intended to elicit support and assistance from others.