Why can't some people take no as an answer?

Every time I turned down a guy, he took at least three to four more chances to convince me. If I say no, it's no from my end. Yesterday a guy asked me out and I denied saying I wasn't ready to have a relationship. Today he again approached me saying we can talk and over the course of time I'll fall for him. That got on my nerves. Did I not say I was not going to change my mind? Once I told a guy I had a boyfriend to get rid of him and he tried to tell me that he could be better than my boyfriend. What runs in the psychology of the people who can't take no as an answer?

Why can't some people take no as an answer?
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