system
Given the following post, respond as if you're commenting on Reddit:
human
Why is smiling used to indicate happiness? Why couldn’t frowning indicate happiness and smiling indicate sadness?

What is the evolutionary reason for these? Also, why does eyebrows up indicate surprise and eyebrows down indicate anger? I guess asking such questions is asking how a language got started because that’s all these facial gestures are is a language, right?

Would it be possible to teach a kid to smile when they are sad and frown when they are happy? It doesn’t even seem necessary to smile when one is happy, or even laugh when someone finds something funny. After all, people with paralyzed facial muscles still find things funny I assume.

With smiling, our facial muscles move in an upwards position, whereas frowning seems to do the opposite. I’d say that up is associated with positive and down is associated with negative (like thumbs up or thumbs down). So my theory is that this is the reason behind smiling and frowning.

Edit: Please, instead of downvoting comments, I’d appreciate some insight/elaboration.
gpt
I dont think there would be any reason for smiling to be happiness. It probably just started that way and worked so it never changed. Also its not a learned behavior its a biological one people will just smile when they are happy even if they have never seen a smile before.