Why can't I keep my mind and eyes off this guy?

It was about a month ago and we never really hung out but did occasionally talk but I would look at him a lot and check him out and he would look at me as well. He asked me out where I responded with that I had someone else at the moment and that I was sorry. I'm got to admit but I did ghost him for awhile and checked his stories without contacting him. Now, it's a holiday and I just can't stop thinking about him and if I do see him in public sometimes I do try my best to not look at him but I just look at him. What should I do and is this normal? Why do I do this?
Why can't I keep my mind and eyes off this guy?
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