BuzzFeed founder Jonah Peretti recently listed some of the reasons behind social sharing:

People share heart warming stories, such as a picture of a card written by a child to his adoptive father;

People share to affirm messages about identity, the reason why many left-handed people chose to share a list of the worst things for left-handed people;

People share content that captures the moment, that “publishes into the Zeitgeist”;

“Animals are not about animals, animals are about being human,” Peretti said, explaining why people share cute dog pictures’

People share very human stories;

People like nostalgia;

People share stories on human rights – “as they make you look good”.”

via 6 lessons from BuzzFeed as it launches in the UK | Media news | Journalism.co.uk.

In addition:

Jonah Berger says:

• Social currency:, It’s all about people talking about things to make themselves look good, rather than bad
• Triggers, which is all about the idea of “top of mind, tip of tongue.” We talk about things that are on the top of our heads.
• Ease for emotion: When we care, we share. The more we care about a piece of information or the more we’re feeling physiologically aroused, the more likely we pass something on.
• Public: When we can see other people doing something, we’re more likely to imitate it.
• Practical value: Basically, it’s the idea of news you can use. We share information to help others, to make them better off.
• Stories, or how we share things that are often wrapped up in stories or narratives.

via: http://knowledge.wharton.upenn.edu/article.cfm?articleid=3206

More external links:

Slideshare deck – Why we share

Berger: Viral’s Secret Formula