I mean what if his/her facial muscles are just fine, but brain cant process smiling activity?
What kinda person can laugh, but not smile.
On the rare occassions that smile happens, it happens only if it happens before the brain realizes what is happening.
Otherwise, even when the person feels like smiling to a certain extent, the brain cant take it to competion by slight voluntary effort.
So when others smile, the person cant smile back.
Over the period, the person becomes phobic to smile, because the moment someone smiles, these things happen simultaneously in the mind of the person:
That person is happy tp see me. (old chum/known person)
Let me smile back.
(a nanosecond passes. no smile appears).
Oh no, smile quick, or the person will take it as a negaitive vibe.
(Another nanosecond passes. No smile, instead, a slightly strained look due to the effort in trying)
The other person's expression begins to involuntarily change on seeing a smile met with strained expression.
(The 'no-smile' person notices and within a flash realises that the inevitable has happened.
The lack of smile is going to cause loss of one more person. The 'no-smile person' feels a silent rage at self for such a limitation. But even though the silent rage does not reach vocal chords, it reaches the face. Now, even though a 'no-smile' person is effectively an alien on earth because of it, the anatomy of the person is not actually so. So in order to express that rage, the person does not have eyes on alien tentacles, that can be moved towards oneself when expressing any feeling for self.
So under the limitation imposed by human anatomy, the eyes of the persojn still happened to look at that other person while that look of anger and hatred filled them. So the other person thought it was meant for him/her.
One more friend lost! Or maybe one more enemy made!