Smithsonian had a good short article on it: http://www.smithsonianmag.com/science-nature/what-emotion-go...
Research shows that amazing good news, like a huge leap forward in cancer research, is even more viral. But unfortunately those happen much more rarely up than someone saying something stupid and getting lambasted.
Ryan Holiday's book "Trust me I'm lying" also really opened my eyes to how a lot of this stuff works.