How come I don't notice when guys check me out?

but when they are checking out my friends it's like the most obvious thing in the world.

like the other day my friend and I were at star bucks connected to a bookstore and these skater guys walk in and they start staring at us and I'm convinced that they are checking out my friend even though she insisted that they were looking at me. I kinda just ignored her and walked off to get some napkins well while we waiting for our drinks one of the guys kept walking pass me and looking at me and I kinda ignored him because I dint think anything of it when we got our drinks and left my friend was totally mad saying that I always do that when guys are checking me out. but I don't notice when stuff like that happens because I'm not use to it. so is there something wrong with me or do other people do that too?
How come I don't notice when guys check me out?
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