For my case, I have perfect pitch and partial synesthesia. The process of speeding up music (usually) also raises the pitch, which not everyone can hear, but I hear it easily and it's interesting to me. Even though it's the same song, the new pitch information and frequency shift makes the song an entirely new experience.
I think a subsection of nightcore fans listen to it because the speed/pitch shift also happens to make the singer's voice higher pitched. Which has the effect of making it sound more feminine or youthful.