What's the evolutionary reason behind feeling 'sorrow'?
I‘ll answer from a perspective I‘ve been studying in Psychology, but can‘t provide sources. If this is not within the scope of this SE, I‘d kindly ask to either add appropriate sources to my answer or close it.

Sorrow is an emotion that has specific cues and expressions like tears, distinct micro-facial cues (which are stable over practically all cultures according to studies of Paul Ekman et al.), a general tendency to withdraw from social settings. 

This can be understood as communicating to other members of your group the need for consolation and social closeness, which would count as an evolutional gain.

Update: 
I found an extensive article on the subject: http://www-personal.umich.edu/~nesse/Articles/Nesse-EvolutionBereavement-2005.pdf