One possible reason could be that Naruto is hiding his face to conceal his emotions. In SasuNaru fanfiction, their relationship is often complex and full of unspoken feelings. Naruto might be afraid that Sasuke will read his true feelings if he shows his face. For example, if Naruto has developed deeper romantic feelings for Sasuke, he may not be ready to expose them, so he hides his face as a form of self - protection.
Maybe Naruto hides his face because he's ashamed or guilty in the context of SasuNaru stories. Suppose Naruto made a mistake that he thinks will make Sasuke disappointed in him. Hiding his face would be a way to avoid seeing Sasuke's reaction directly. It could also be related to some power - related incident. For instance, if Naruto has tapped into a new, dangerous power that he can't fully control yet, he may hide his face to keep Sasuke from getting involved or worried.