According to the New York Times, recent research conducted separately at the University of Texas and the University of Pennsylvania, USA, has demonstrated that emotion is the key. The more intense an emotional response a piece of content provokes, the more likely it is to be shared. Strong positive reactions are especially likely to have this effect.
Taking things one step further, a marketing technology company called Unruly has identified the psychological responses that increase the chances of a video or content going viral. These include happiness, exhilaration, amazement, inspiration, hilarity, contempt and disgust. In order to be successful, the content has to elicit a response powerful enough that people stop what they're doing and feel compelled to share it with their network.
Unruly has also demonstrated the importance of timing when it comes to viral advertising. The first three days after content is published are make or break for its success in going viral, and it should be broadcast to as wide an audience as possible during this critical window of opportunity.
Sometimes people share things just because they want to influence how others perceive them (even if they haven’t read/watched the items themselves). This behaviour is about individuals projecting a desired identity to their peers.