system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people cry when they're sad/happy?

Is there any purpose/biological reasoning behind it?
gpt
We are social animals.  The biological NEED for tears is simply to keep a smooth sterile surface for us to see out of.  
Somewhere we developed an emotional link.  It makes sense for social animals.  When you show outward signs of distress (crying, yelling, cringing), the others in your group will respond by trying to identify the cause.   So crying works to get help removing the bad things and to share the good things.