Why would a guy from charming and friendly towards a Girl to serious and Distant?

This guy in my Friend group used to be very friendly with everyone including me. He still approaches me occasionally but acts very serious (almost frowning) only around me and Never smiles to me. With everyone Else He is very relaxed.

He Acts like I slightly offended him But Im pretty sure I haven't since I was always polite to him and we don’t Communication that often. Do you have any idea why a guy would change from being chatty and friendly to Cold and Distant with only one person?

Why would a guy from charming and friendly towards a Girl to serious and Distant?
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