Why do guys put down other guys?

So when a woman gushes over a man, in front of another guy I have noticed time & time again they will look for and point out any little flaw about the guy. Other than feeling insecure, why do guys do that? Or is it just based off feeling insecure?
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Edit. By flaw I mean both physically and/or their personality
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Wow thank you all for the answers, I never really thought about it that way but it makes total sense and now I have a better understanding. I've had male friends do this and/or complete strangers or guys I barely knew
Why do guys put down other guys?
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