I’m going to go with the others and guess it’s difficult when you’re trying to smile on command—like for photographs. Smiling on command can be difficult but you can practice getting it down.
Focus on your eyes. Next time you find yourself caught in a laugh or a spontaneous smile, try to notice how your eyes feel. Notice what muscles are pulling. Notice how your eyes feel—maybe they feel softer, and the bottom lids push up just a little, and your eyebrows relax and drop down, and the outsides of your eyes crinkle. I think of my eyes in a smile as warm eyes, or bright eyes. And when I’m smiling on command, that’s the part of the smile I find first, because that’s what comes first in a spontaneous smile. Once you get your eyes in “place,” you’ll notice that the corners of your mouth naturally start to pull upwards, and there’s a muscular space for your mouth to open in a grin, and suddenly the smile that had initially felt forced starts to fall into place.
Another good strategy is to try and psych yourself up in the countdown before the picture. Look down and imagine you feel a swell of happiness, and let that feeling pull your eyes into a smile which pull the rest of your face into the smile.