Why do I still think of guy I never dated?

I was friends with this guy and we never dated though he knows I like him and he likes me too but we never dated until he stopped logging on his social media and I wonder what happened to him? we never spoke since July last year and I still think about him sometimes I think he might met someone and he's dating someone else now other times I think he might be dead I can't stop thinking about him and I dont know what to do?
Why do I still think of guy I never dated?
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