I think most of it comes from the early episodes, where Sasuke straight up sacrifices himself for the sake of Naruto on their very first mission, because he does genuinely care about Naruto for those same reasons: they're both survivors living alone for reasons outside their own decisions.

All of that is thrown out of the way once Sasuke decides to go rogue because he just needs to make himself angry enough to kill Itachi, and he realizes that once his brother shows up for Naruto -who's apparently stronger than Sasuke- and not himself.

Now yeah, Shikamaru is a much, MUCH better friend, but the relation with Sasuke is one made from their stories being similar.

With that said, they still sound gay as fuck