How can someone be so expressionless?

Heard about exciting/happy/good news, expressionless. Heard about jokes or funny things, expressionless too, like holding in the laugh to keep a straight; flat face. Only shows expression when upset and angry. Their face is so cold and boring that i dont even want to look at it when im talking to them, it makes me mad (on the inside) because their face is lowering my mood. Do you know or ever encountered with someone like this? That you prefer to look at a pig than look at their expressionless face.

How can someone be so expressionless?
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