Male models generally don't smile because looking fierce gives the man dominance and status, which are usually characteristics of the stereotypical alpha male. Movie posters, therefore, very rarely have smiling faces, unless it is a comedy (even so, sometimes the main characters are doing something stupid on camera but not smiling). Case in point: would girls be as attracted to Jacob and Edward if they looked like this? Bella looks like the dominant one and the guys look like pussies!
There is a case for smiling generally in on-editorial men's spreads, and that is, it increasing the model's likeability factor. Case in point:
If you are naturally more beta or less dominant, look more fierce in photographs to convey status. If you are naturally already very buff and strong-looking, a smile can add a nice touch of gentleness and charm to your already alpha look. Learn to play with your expressions as you shape and craft your public image!