It's great how we're made. The natural aggressiveness our species needs to survive has always been measured against the compassion we feel.
I think it's fantastic that we're made in such a way that doing something for someone else makes us both healthy and happier (see http://ca.geocities.com/earthpages5@rogers.com/gp1.htm?rss)
It shouldn't be a surprise that we're more altruistic when we laugh. When we feel good, we want others to feel good.