Firstly, bit of backgound on the nofollow attribute:
In 2005 the search engines huddled together in a dark room (we have a wild imagination) and created the “nofollow” attribute. This attribute was created to fight comment spam.
The nofollow attribute is added to a link to tell search engines not to pass any value/credit to that link. Here is an example of how its instrumented:
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At that time forums, comments, user profiles were full of spam with automated tools such as Scrapebox being used to build links through comment spam.
The search engines responded by implementing the nofollow attribute. Webmasters then begain enabling the nofollow attribute to any user generated content (i.e. comments/forum posts/user profiles/bios). This meant that any comment spam wouldn’t pass any link equity/credit and this strategy of black hat SEO was stopped in its tracks.
Not only was this spam impacting Search Engines link graphs, it was really annoying for site owners (and us!).
The dofollow is the opposite, and this is assumed and it isn’t necessary to specify that a link is dofollow.