Adding nofollow to a link means you, the site owner, aren't sure if the link is trustworthy. For instance, most blogs that allow user comments to include links make those links nofollow. That's because the links are added by an outsider, so the site owner doesn't want to say that such a link is definitely trustworthy.
When you, the site owner, add a link to an authority site within a blog post of yours, then you certainly think that link leads to a trusted website. If you didn't trust the site, why would you link to it? You're specifically, intentionally telling the reader that "Here is an authority site that acts as a good source of information about this topic." So, why would you, the site owner, make such a link nofollow?
I add authority links to my website articles - I think Google likes that. And I never add nofollow to those links. I don't want to tell Google that I'm not sure about how trusted the link is!
