Quote:
Originally Posted by Dengar
The "real" trigger probably has to do with the user's mental state
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In this chapter Tobi states that Kakashi "lived a life of regret". When you kill your best friend you regret it. This "life of regret" may be what allowed Kakashi to unlock it. Kakashi probably blames himself that he couldn't save his father, his sensei, Obito and Rin. And also Sasuke, since he was Sasuke's sensei.