Short answers, pulled from the story.
Sadness activates the middle and posterior temporal cortex, lateral cerebellum, and midbrain. Researchers observe this biological signature using positron emission tomography. The bilateral inferior and orbitofrontal cortex fire in unison along with the putamen and caudate.
Pediatrician T. Berry Brazelton observed that enforcing rules against sadness creates adults who are shallow and manic. When mothers cannot allow minor distress during separation from early symbiosis, children fail to learn to deal with sadness independently. This prevents the development of the ability to process the natural ebb and flow of human emotion.
Sadness promotes cognitive changes that restructure beliefs and goals to help people cope with loss. It also signals a need for assistance by eliciting support from others through group norms and physical or verbal expression. This shared experience decreases emotional polarization and increases relationship building.
A sad facial expression with small pupils is judged to be more intensely sad. A person's own pupils mirror this change by becoming smaller when viewing sad faces with small pupils. This mirroring of pupil size predicts a person's greater score on empathy.
Edmund Spenser in The Faerie Queene endorsed sadness as a sign of true spiritual dedication. J. R. R. Tolkien distinguished sadness from unhappiness in The Lord of the Rings as a settled determination. Julia Kristeva argued that refinement in sorrow is the imprint of a humanity that is subtle and ready to fight.