Why are we told not to smile for passport photos?

People don't NORMALLY walk around smiling, and the passport photo must show you as you are normally, and because there are different degrees of smiling (from a slight smirk to a full smile with teeth showing, on the verge of a laughter) that CAN make you look different. the idea is for people to use a facial appearance that is easy to recognize the person (you); it's the same with government official photos for ID. They do not want females using excessive make-up or after they come from a beauty salon. Those are their rules, so everyone can look uniform, natural and easily recognizable as they normally are when facing a government official comparing you to your Photo ID image. Besides, Photo ID images are not meant to be glamour photos.

It has to do with being able to recognize a person to the photograph when you smile you may look different then when you don't.

General rule, it distorts the face.

Because passport size photos use in official documents and look like a gentleman and gentleman shows attitude not smile.

There are different levels of smile with some even bordering to laughter. When your mouth is shut, you always look the same that's why passport photos are taken with a neutral expression for easier identification by the immigration officer at the port of entry.

Because if you look happy then the illuminate can get u good