cultural symbionts. Many memes promote cooperation partly because they need extended contact between humans in order for them to reproduce themselves.
One interesting illustration of the symbiont hypothesis involves Toxoplasmosis. Toxoplasmosis is caused by a microorganism that makes mice love cats - and is transmitted from mice to cats. It turns out that Toxoplasmosis also makes people love cats - and might be contributing to the plague of cat memes on the internet. Toxoplasmosis is an interesting example of a microorganism promoting love - partly because it is so well known. It makes a useful illustration of the symbiont hypothesis.
Update: for a sceptical take on the topic see here.