Why do some guys hate being called nice? I told a certain guy he's kind and nice but he strongly disagreed with me and said there are many guys nicer than him.

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Because they’ve internalized the common “wisdom” that ‘nice guys finish last’. Unfortunately all too often the only way you can convince them otherwise is by allowing them to finish first. With you.

Though in your case he may be actually trying to make a logical point. Not a very good one, I might add.