I want to share something with my friends because I think it might inspire them or make them happy. Or, in the case of news, let them know about important stuff that's happening in the world. They do the same thing when they share with me.
So to blindly share everything just creates a lot of noise. If everything is shared by default, you'll need to start sifting out the signal from the noise. Which is annoying because I trust my friends to only share stuff that's relevant.
We all love lolcats but know how to find them on our own.