Why are some introverted people so unexpressive and hard to read? How to relate to people like that?

Anonymous
For some women who are introverts, it's hard to gauge whether or not I'm having any positive effect, because they don't smile much, and have poker faces. I've had shy friends that said they had to work on being expressive and smiling, and people thought they were really serious if not a party pooper. Is it 'cause shy people are sensitive and have a "poker face" to hide their emotions?

What is the best way to relate and talk to people like that? I obviously don't think they want someone bouncing off the walls, all up in their face. But they probably don't want a serious, cold approach either.
Why are some introverted people so unexpressive and hard to read? How to relate to people like that?
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