Why does a guy watch me walk away?

this is a very serious question so please take it seriously

so there’s this guy that I think is cute and i stare at him a lot but ended up having a girlfriend :/
but I think he knows that I’m starting at him but he does stare at me when I’m not looking but when I look at him he doesn’t look away

today when I was going up the stairs he was looking at me while he was going to another direction and he was looking at me without looking away when I caught him and he was kinda walking backwards

Did he like me before he got a girlfriend?
Why does a guy watch me walk away?
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