Sadness is an urge when an object of love is lost.
Sadness is also called sorrow, woe, or grief.
Sadness is an emotion which branches out from love's disappearance.
We feel sad when we lose a family member, or are apart and unable meet anymore. Sadness is the loss of the object of love. We feel sad when we lose the object of altruistic behavior.
Sadness has a special facial expression. The inside ends of the eyebrows are raised, the eyes are narrowed aslant, and both ends of the mouth are lowered. Also the person typically cries. Tears are the main expression of sadness. Strong sadness makes them crouch down on the spot. Their physical movements slow down, which accompanies the stagnation of various activities.
The act of crying is a signal that calls for others to come when something they cannot deal with alone occurs, so it occurs not only in sadness. Crying when sad is a message telling everyone that they lost their object of love and that they need a new object of love.
The stagnation of various activities carries out the function of making the person reappraise and reflect on their lifestyle and change their behavior patterns. This is because something bad happened, so this is done to avoid it happening in the future.
Sadness is unpleasant, so we make efforts to avoid sadness. The cause of sadness is the loss of the object of love, so we make efforts not to lose the object of our love.
Also the person that is currently sad empathizes with other's emotions well. This is in order to find a new object of love.
Therefore, it is an instinctively correct decision to court and win the heart of someone who is crying because they are sad at being dumped by the object of their love. From instinct's point of view, this is the purpose of sadness. However, since this is a genetic pattern and not that person's will, the possibility of being disliked is high.
If a person doesn't feel sadness then he or she won't cry, and cannot get assistance from others which is an effect of crying. That person is strong, but gives the impression of being unapproachable. On the contrary, those who easily feel sad and cry are thought to be undependable and gloomy people.
Sadness developed in humans because we can get the aid of our fellow humans by displaying sadness. We can assume that animals which form groups and live together can feel sadness. However, we can assume that birds, reptiles, and fish don't feel sadness. They can be seen doing strange things when they lose their partner, but this seems to be just confusion, not sadness.