What Is Sadness's Purpose?

I just finished watching Disney/Pixar's Inside Out for the second time, and now that my Mom has had a chance to see it to, I feel free to blog about it. The trouble is that many of the blogworthy thoughts inspired by the movie are complicated. For example, the movie is about emotions, and Joy introduces the cast by telling us what each emotion's job is. Fear keeps us out (or at least cautious) of dangerous situations, Disgust keeps us from getting poisoned, and Anger tries to keep things fair (mostly by complaining about unfairness). However, Joy couldn't think of what purpose Sadness might have, and I'm surprised to find that neither can I.

In the movie, and in real life, one of Sadness's talents is expressing compassion, which sounded like a pretty good purpose to me, until I realized that the purpose of compassion is mostly to help someone overcome or at least cope with the sadness they feel. If the purpose of sadness is to inspire compassion, then it could be said that the purpose of sadness is to help reduce sadness, which seems contradictory. There must be another purpose for sadness.

Perhaps one purpose for sadness can be found in the dichotomy between sadness and joy (which drove much of the plot of the movie). There are a few scriptural references to the relationship between joy and sadness in the Book of Mormon. The first one that comes to mind is 2 Nephi 2:23, which suggests that experiencing joy wouldn't be possible without also experiencing misery. This could make sense, given that we can't truly appreciate something unless we've experienced it's opposite.

However, I still wonder if sadness may have another purpose than just helping us reduce sadness and feel more joyful in its absence. Perhaps I'm being sentimental, but I think that Sadness must have some positive, productive purpose. There has got to be something good that sadness does for us, beside helping us console others and appreciate experiencing joy. But if sadness has that kind of purpose, what is it? Is there a proactive reason we experience sadness, or is helping us reduce sadness and enjoy joy supposed to be reason enough? I wish I had an answer to those questions, but I unfortunately don't. If sadness has its own purpose, I don't know what it is.