Emotional content — especially positive emotion — is the main propellant of social sharing, reports Natalie Kitroeff in The New York Times (5/20/14). "People share things they have strong emotional reactions to, especially strong positive reactions," says Rosanna Guadagno, a social psychologist, who bases her observation on a study of 256 people. This goes for videos as well as "status updates," which are more contagious when they are upbeat, according to research by Facebook, Yale and The University of California.
It’s not that sad stories can’t go viral; it’s just that they must "give you something to do with that sadness," says Daniel Mintz of Upworthy, "a site for shareable content." For example, the "video of a boy who died of cancer, but not before producing a hit song and performing sold out shows" had a positive aspect. The video’s title, ‘This Amazing Kid Got to Enjoy 19 Awesome Years on This Planet. What He Left Behind is Wondacular,’ (link) is also credited with fueling some 20 million views.
One’s physical state can also affect behavior. A study by Wharton’s Jonah Berger and Katherine Milkman found that subjects who jogged in place briefly were more likely to forward or ‘like’ something than those who jogged and then rested. "Arousal is an aversive state, so people want to get out of it by sharing," says Jonah. Meanwhile, recognizing the ‘ego-factor’ in sharing, media companies now "package content that will make each member of the masses who disseminates it burnish an online reputation while feeling, oddly, unique."