Why some people never smile?

According to research higher testosterone makes a person less likely to smile. This is one reason why women smile more than men. 

According to studies smiling can be used as a signal of submission in some situations. Those who don't smile might not belong to the submissive type.

People who don't smile often might be less agreeable than those who smile often. Smiling is often used as a method for communication and those who don't smile often might not be willing to communicate with others.

People who don't smile often might be less agreeable than those who smile often. Smiling is often used as a method for communication and those who don't smile often might not be willing to communicate with others.

Some people try not to smile in order to act tough or to seem stronger. Those people might have learned during their upbringing that smiling makes a person weaker. (See  How to differentiate between a genuine smile and a non-genuine smile?

People who are not happy with their lives or those who are emotionally burdened might find it very hard to smile or to laugh. 

People who were emotionally hurt in the past or even bullied might choose not to smile in order to protect themselves from further damage. In such a case, not smiling is considered a defense mechanism. (See Why Eminem never smiles?)

Some people don't smile often because they think they look bad when they smile. Believing they have bad teeth is one example that could make a person smile less.

Some people might chopse not to smile in order to be taken more seriously. Those people might believe that smiling might not make them seem serious.