Why do some guys look away quickly when I look at them? Do they think I’m unattractive?

I've noticed if I'm just out somewhere that some guys quickly look away when I see them. I will be on my phone or something and when I look up they immediately look away. I'm just curious what makes them look away so quickly!
Why do some guys look away quickly when I look at them? Do they think I’m unattractive?
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