How do some people never smile or laugh?

Anonymous
I have a couple of coworkers who never smile or laugh. The most they showed was a really tiny and brief wry smile, which I don’t count as a genuine smile (as if they’re just saying “who would’ve thought?”). They did it only once after hearing some jokes during our informal staff lunches. (And we actually have some comedians at work who always make jokes).

My non-smiling coworkers are both men, and each considers himself to be a tough guy. One is ex-military and the other is a devout cross country runner and coach.

I’m a guy and I smile and laugh. I think it’s healthy and a normal reaction, especially if the jokes are appropriate. (Don’t get me wrong, I can be really serious and tough at times too). But how is it that some people never smile or laugh? Are you or do you know anyone else like this? Is it just your personality? Is it a learned behavior? Does it take some conscious effort to not smile or laugh?

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Thank you all for your thoughts.
How do some people never smile or laugh?
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