Why did he reject me?

I was really into this guy and decided to tell him how I felt through messenger and he responded. In his response he said I think you're really pretty but Im just not super into you. I know I shouldn't dwell on it and I'm getting over it but I can't help but wonder if that was a genuine statement or if he was just trying to be nice? Why would he reject me if he thought I was really pretty? Why wouldn't he give it a chance?
Why did he reject me?
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