He keeps going after Sasuke even though Sasuke keeps pushing him around, hes too scared to let go and accept what Sasuke has become.
As for the series itself, i agree that some other characters are somewhat left out, but would u want 20+ filler episodes of every side character training?
One piece does a good job of pushing the level of its characters up without bringing in too much attention to training - but there are also about 30-40 less character to worry about, i think with the coming of the war some of the other ninja in Naruto will probably show off some new abilities/hype but they wont be as powerful as Naruto and Sasuke,
just how Luffy (and the boss enemies that he faces) are always a LOT stronger than the rest of the crew.