Why is it at this time of year, everyone breaks into "give 'til it hurts mode"?
In this article: http://www.telegraph.co.uk/connected/main.jhtml?xml=/connected/2006/12/20/ecnxmas20.xml an idea of "upstream reciprocity" is explained.
Here's a taste:
"Our analysis shows that gratitude and other positive emotions, which increase the willingness to help others, can evolve in the competitive world of natural selection." Scrooge himself would have been impressed by their arguments which show how, even in a nature that is supposed to be red in tooth and claw, a person who has just received help may go on to help several others in "an 'epidemiology of altruism' resulting in an explosive increase of altruistic acts."
I'm not sure this explains goodwill during the holidays specifically, but I found it hilarious that scientists are even trying to explain this. Gotta love these guys!
Happy paying it forward day!