Why a guy becomes quite when I talk about a guy I am talking to?

I know a guy for the past 6 months and we are friends, but he has been sending me mixed signals to the point I am so confused. We are both from different cultures and he is very interested in my culture. He blows my mind with the details he knows about my culture which I guess he has been doing a lot of research. He pretends to be confident but he is very quite and shy around me, he sometimes ignore me when we are alone but seeks my attention all the time when we are around other friends. I was talking to him ( lets call him A) about the guy I am talking to ( lets calls him B) and I was telling him how it went, he was excited to hear the details but went quite when I was talking more about the B’s traits and what he does for a living.

I got confused, because he kept bringing up B story all day and was telling our other friends about my adventure with B.

what does it mean?

Why a guy becomes quite when I talk about a guy I am talking to?
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