why models never smile when on stage?

The definition of beauty seems to change every day it seems. Everyone's attention needs to be on the outfit the model is wearing, not the model. The model needs to remain neutral in order for the outfit receive as much attention as possible.

I am not a model or anything but here is what I think I think that they are there to sell the clothes not themselves any type of facial expression will draw attention to them and away from the clothes they are modeling

I have to agree they need to draw attention to the clothing and therefor they aren't allowed to smile. I agree it would look more kind and people do get a good feeling by seeing other people smile, so perhaps that would motivate them in buying more stuff (because of the good feeling). I think this clear look is also considered sexy in some way, it's a bit of a dominating look 'here I am, want some?'. I am personally not a big fan of it, but they kind of need to preserve the image of being queen of the world and owning the catwalk, so I think theres your answer.

the model's job is showing beautiful clothes when they are catwalking but not showing their pretty faces. designers can not stand the limelight which belongs to their works is snatched by the model's charming smile.could you guess why models are all very bony,just like clothes-rack?it's on this account, too.