Upon becoming a shinobi, one must abide by specific rules when out in the field. Ninjas follow this code in order to "kill their emotions," preventing them from running wild and causing the mission to fail. The most important thing for a shinobi is to be a tool for achieving their village and country's goals. Emotions are considered unnecessary things.
These rules are taught to young ninja when they enrol in the Academy.
- 4. "A shinobi must always put the mission first."
- 25. "A shinobi must never show emotion."
- "A shinobi must see the hidden meanings within the hidden meanings."
- "A shinobi must never question their commander."
- "A shinobi must prepare before it is too late to."
- "A shinobi must never show attachment." 
Breaking the Rules
While many shinobi follow the rules and strive to become emotionless, they are still human, and may not be capable of completely killing their emotions. Nor may this be ideal. Minato explained to his students that while the rules and regulations are important, they are not infallible. A ninja must also know when to bend these rules in order to adapt and cope with the situation. For Kakashi, his belief is that "while those who break the rules of the ninja world are trash, those who abandon their comrades are worse than trash", a belief that derived from his team-mate Obito. In Naruto's case, he does not believe strictly following these rules will make him a true ninja, and has decided to be a ninja in his own way, by never giving up.