Why do guys call girls ugly even it they're not?

Anonymous
Ok, I know that when some guys think a girl is ugly they will be a jerk and just tell her to her face. But I've also heard that sometimes guys will flirt like this by calling them ugly. And sometimes they're just joking around. I just don't get how to tell the difference between when they mean it or not. Usually I don't see a guy just straight up tell an ugly girl that she's ugly...but then again there are guys that do. Guys have told me I'm ugly before. One said, you're so ugly. But that was in the third grade. Don't boys do that to girls they like at that age? Then when I was in junior high, two guys used to constantly call me ugly. But the thing is, is that they would say it with a smile and stare at me all the time. So that sounds like they were flirting. So I'm not sure why guys act like this. But is it true that when they're young they act like that towards girls they like?
Why do guys call girls ugly even it they're not?
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