ELI5 Crying - why do we cry with both sadness and joy?

Crying functions as a biological reset button that activates the parasympathetic nervous system to restore calm after intense emotional arousal. This physiological response is identical for both extreme sadness and joy because the body reacts to the intensity of the stimulus rather than its emotional meaning. Beyond internal regulation, crying serves as a vital social signal; visible tears and blurred vision communicate vulnerability and non-threat, increasing the likelihood of receiving communal care. Physical side effects, such as the "globus sensation" in the throat and involuntary diaphragm spasms, result from the body's struggle to manage oxygen intake while under stress. Unlike reflex tears caused by irritants like onions, emotional tears contain higher levels of stress-related substances, though their primary value lies in triggering a shift in the nervous system. This hardwired mechanism is essential for survival, beginning with a newborn’s first breath and evolving into a sophisticated tool for human bonding and hormonal balance.