Why was he looking at my lips?

I was beside a friend of mine ( male) and I started talking to him. As soon as I started talking, he looked at my eyes very briefly and then directly at my mouth. He stared at my lips for some seconds but It was very weird. He did not look disgusted ( like If I had something stuck on my teeth or something like this) and he was looking with a serious look. He looked at my eyes quickly, my mouth ( for more time), then my eyes again and finally my mouth before establishing eye contact normally and looking to the side. Does It mean anything when a guy looks so much at your lips instead of your eyes? Was not he paying attention to what I was talking about? To be clear, he had a normal/ serious look in his face, he did not look disgusted or had a "lustful look"
Why was he looking at my lips?
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