I tried looking up an answer to this question, but all I got were links to depression hotlines. Gotta love Google :)
Why is sadness as a separate emotion beneficial? Sadness seems mostly to be associated with loss, or a severe change in one's future. Not getting a job, losing a loved one to death or breakup, regret for a decision made or event occurred that can't be undone. It seems in all of these cases that simple pain would suffice just as well, and yet there exists a dedicated emotion for it.
What is the benefit of feeling sadness as a distinct emotion from pain?