To gain better insight into what makes people share content online, somebody studied the emotions associated with viral marketing campaigns.
If this subject is your bag then this blog from Harvard is worth a look. Personally, I am not convinced. There are some really nice charts but I was left with the feeling 'so what'.
I think the thing that made me most sceptical was the use of the imagery from imgur (see above). Personally, the only emotion I felt was one of bordom. Dick Stroud