But I wanna know one thing, when did I become a ghost?

from Kid Cudi – Ghost! Lyrics on Rap Genius

He’s wondering why he feels like such an outcast. He often refers to this in his songs by saying he’s from another planet (i.e. Embrace the Martian, Do It Alone, etc.).

Another Idea is that it is not a reference to becoming an outcast, so to say. He is wondering at what point he became a shell of his former self: A lonely relatively unknown stoner who had his real friends who loved him for who he was, to the man who he is today: a famous, (at the time cocaine addict) who lost sight of what matters in life for a life of superficial gain and people who only care for him as a celebrity.

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