Where does sadness originate?

Is it possible to properly generalize an answer to my question?

There is much pain and suffering in the world, and we see a fair dose of it here on Fluther. Some of it stems from basic needs going unfilled: lack of water and food. There are some types of sadness that seem more about relationships of any type, such as romantic or familial. Some sadness seems self-imposed, or at least the suffering seems so.

I’m not sad, and I’m wondering about the nature of the emotion. I have experienced clinical depression in my life, but that is an illness and not the same thing as sadness.

Please, note this is a General Section question. Thank you.