Why would a guy still be interested in you when you keep ignoring him harshly?

I ignore him completely to the point where it's borderline rude. I also have said to him that I don't want to talk to him and he's very clingy with me in front of everyone. Still he continues to be interested in me and keeps trying to talk to me. What's up with him?
Why would a guy still be interested in you when you keep ignoring him harshly?
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