People like to be liked and because of that, most people do share....
The key is that they share (which is an unselfish act) for human reasons which tend to be selfish.
Now I want to be clear - selfish is not meant in a dergoatory way. I mean selfish in the fact that they share because they want an outcome, that outcome might be an unselfish thing....so they share their wealth to improve lives of others...but it is to fulfill an underlying need in them.
So when it comes to social media - sharing which is part of the DNA of the web, needs to be seen as more than a retweet, a link. It is an expression of what that person wants to achieve for their own contentment.
Here is a graphic I use when coaching on social business models....
We need to give a reason to share - that is different to the reasons why we want people to share.
If you want to influence human behavior you need to appeal to both heart and mind.