Guy's Behavior

Why would a guy be staring at me when I wasn’t looking with an annoyed / frustrated expression?

Context: he’s a PT at a gym I go to we grew up together in school, have been going to that gym for 6 months he has worked there for 3 weeks, I followed him and some other people who work there on socials, he didn’t follow me back but when I wasn’t looking I seen him staring at me deeply like he’s annoyed, (I wasn’t exercising just talking to my female coach) does it mean he hates me

Why would a guy be staring at me when I wasn’t looking with an annoyed / frustrated expression?
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