1. Character development:
It is because of the awesome character development they have focused upon, during the whole series so far. Every main character portrayed in Naruto series, has his/her own story. They don't just come and go away. Every character has a dark past behind what they are today. This character development, binds the viewers with the story. They grow along with the character.
Even the villains/antagonists portrayed in this anime series, are likeable because they all have their own well-justified reasons choosing the darker path of destruction and hatred.
Example: Kabuto, Obito