Why Eminem never smiles?

Some people theorize that Eminem rarely smiles because he wants to reaffirm the messages present in his songs. Many of Eminem songs are angry songs and thus smiling more often might make him seem like saying things he doesn't mean.

Some people assume that by not smiling often Eminem can achieve the goal of projecting the image of toughness he wants to project (See also why many people like Eminem).

Some people say that Eminem is tough that he doesn't like to smile often.

Some people claim that Eminem doesn't smile a lot because he has been through too much in his life. Those people cite some of Eminem lyrics as a proof to their claims.

Some people claim that Eminem believes that he looks better this way and that's why he doesn't smile much (See also Why some people don't look good on photos).

Some people claim that Eminem doesn't smile much because it's a part of his personality. Eminem is a rebel, competitive and sometimes angry person. Smiling a lot might not fit with that personality cluster.

Some people assume that Eminem smiles normally but people just focus on the pictures where he doesn't because of his angry songs.

Some people claim that Eminem might not want to appear smiling because of believing that it makes him weak (See also the difference between a real and a fake smile).

All of those are theories people made with no real evidence to support any of them. They are best effort guesses.