Why did he reject my offer to buy him coffee?

I have a guy friend from college and we both graduated, started working, and reconnected recently. We met up somewhere to catch up a few months ago and he bought us our coffee. We recently met again a few weeks ago and i offered to buy us coffee and i didn’t try to come off as flirty but me and my friends usually take turns treating each other. My guy friend had a very serious face and said “no, it’s okay.” Did he think i was trying to flirt or something?

Why did he reject my offer to buy him coffee?
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