https://www.psychologytoday.com/us/blog/fulfillment-any-age/201804/its-not-easy-diagnose-personality-disorder
To begin with, emotionally crying appears to be a uniquely human behavior. Animals shed tears as part of a normal ocular functioning, and there have been anecdotal reports of animals shedding the occasional emotional tear, but for the most part only humans routinely cry out of sadness and other complex emotions. Interestingly, humans cry in the context of certain stereotypical emotions as well as their apparent opposite, or “counterpart,” feeling.1 For example, we might cry out of grief at a funeral with the death of a loved one, but also with elation at the birth of a newborn child. We’re likely to cry from heartache when a romantic relationship comes to an end, but we’re just as likely to cry at a wedding as we witness the forging of a new bond. The emotions we experience at these moments are difficult to put into words and often go beyond “happy” or “sad.” Perhaps crying helps us to communicate what we’re feeling in a way that language cannot.
Indeed, in trying to elucidate the purpose of crying, researchers like Dr. Vingerhoets have focused on both it’s “intrapersonal” functions (the effects of crying on the individual) and its “interpersonal” functions (the effects of crying on other people). It has been suggested that the interpersonal aspects of crying might in particular account for its uniqueness to human beings by virtue of its evolution within a range of social behaviors that have made us successful as a species. According to this view, crying is primarily a form of non-verbal social communication aimed at eliciting assistance, comfort and social support from others. Research to date has shown that when people see other people crying, they clearly recognize it as a reliable signal of sadness or distress (in a way that’s more convincing than words) and that typically results in feelings of connectedness and responses of sympathy and a willingness to help from others.