We recommend making sure all internal links on a domain are marked as follow links. Search engines interpret internal nofollow links as a signal that content is being hidden or unserious.

Additionally, your link juice will get lost. So the energy of the linking page will not be forwarded to linked page but still be taken away from the link source.

If you use nofollow tags for comments, you should adjust this as well. Previously, nofollows were used to prevent abuse through link spamming (e.g. when the comment section of a serious website is spammed with links to smaller websites). Meanwhile, Google has introduced a new tag for this case, which can be implemented with the code rel="ugc". (Google source)

Here is an interesting link about Common SEO Mistakes and How You Can Fix Them.

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