Steven Stosny was the keynote speaker at the Bluegrass Healthy Marriages conference. Stosny is a marriage therapist and researcher, and co-author of the wonderfully titled How to Improve Your Marriage Without Talking About It (which must be aimed at men).
Stosny had a fascinating sociobiological tidbit in his talk. He said the portion of the brain that is given over to anger is more than twice as large as the portion concerned with self-preservation. His reading of this: we developed anger to get us to protect our loved ones. We need to protect our loved ones because humans are more dependent on other people than most species are, especially during our long childhood. It is to our long-term advantage to protect our loved ones, especially our genetic relatives. But protecting our loved ones to the extent of endangering our own self-preservation is a hard and biologically costly choice. Thus we need to devote big brain resources to overcome our short-term individual interest in order to realize our long-term social interest. And anger makes us mad enough to override our higher calculations of interest and just do what needs to be done to protect our own.