Some scientists think that crying is a response to pain, either physical or emotional. Other scientists have a theory that crying relieves stress by reducing the hormones in the body that are associated with stress. I like that theory because it means that a good cry might actually be a useful and reasonable response to stress. That would remove some of the stigma associated with people who end up in tears in public.
Recently some scientists reported that the “smell” of a woman’s tears reduces a man’s sexual interest in her. Its quite a controversial finding and needs more work before it will be widely accepted. But if it is true then it might suggest that tears evolved as a chemical signal to other members of our social group.
We also cry out of joy and when we feel touched (that’s why people cry at weddings). So it’s not just out of sadness and pain.
I think that tears act as a physical signal to other humans that we are experiencing extreme emotions. If the research that Darren has shown is true, then tears also contain chemical messages that other humans can pick up.