Well here we are with another generalization about models. There is no one way models take pictures. It is up to the person who decides what is shot. Not smiling is considered to be more edgy. This is a more common look in fashion modeling that in any other sector of the modeling industry. Commercial models almost always smile as well as print models for catalogs. Now if you are speaking of photos on the runway, you don’t see the models smiling because for one it is not needed, and as Allie Hamilton mentioned, it is to not attract attention to their faces. One other thing is that when someone is smiling in a picture it looks to fake and planned out. Where as looking away from the camera and not smiling give more of a real life perspective rather than a posed simulation.Compare the two images of Cara Delevingne; Which looks more intense and candid? To me the one on the right looks as if no one else is around. As if we caught her candidly where as the one on the left looks posed.

Source: Why don’t fashion models smile? – Quora