Guy's Behavior

He stops at mid conversation just to stare at me. Why does he do this?

He will be talking to his friends and I will notice him just stop turn around and start staring at me from head to toe until I finish walking pass. We used date but not officially we got into an argument and just stopped talking. But he always does that and he follows me around like literally inside the supermarket or whatever. We don't talk though. Why does he do that?
He stops at mid conversation just to stare at me. Why does he do this?
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