Am I the only one who gets this?

Quite often if I see a couple who is around my age the girl gives me dirty looks and kind of glares at me. I never ever flirt with any guy who I know has a girlfriend so it's not that, I barely even look at him unless it's rude not to. Why do they do this? I know I'm a girl but I just don't get that mentality it makes no sense.
Am I the only one who gets this?
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