Masashi Kishimoto said this quote

To them I'm simply an object from the past that they wish will disappear Then why do I exist? Why am I alive? When I thought about this I could find no answer. But as you live you need a reason otherwise it's the same as being dead, I then came to this conclusion I exist to kill every human besides myself. Fighting only for yourself living while only loving yourself If you think that everybody else simply exist to allow you to experience that feeling nothing is better then that world. As long as there are people in this world for me to kill and continue to feel that joy of living my existence will not vanish.