Why would a guy become obsessed with a woman who he doesn’t want a relationship with?

I was seeing this guy. He didn’t want a relationship with me. I break things off and he starts with this weird obsessive behavior (calling me from different numbers when I block him on everything, cornering me and following me around in person asking to talk and wondering why I’m avoiding him, etc.).
Why would a guy become obsessed with a woman who he doesn’t want a relationship with?
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