I can't really change your view on this, but give some food for thought. I think many people smile out of social pressure and not happiness. I don't think trying to smile is good or healthy in the long. Smiles should come naturally, and when they don't, people shouldn't "fake it 'til they make it".

I think there is a huge stigma against negative moods in many western societies, it's to the point where it's simply not acceptable to be unhappy, sad or depressed. I suspect that this, paradoxically, stresses people out and leads to less happiness.

Anyway, have you tried relaxing and being fully accepting of whatever facial expression comes naturally? I.e. if you're sad/angry/annoyed, don't put in too much energy into hiding it too much. I think bad moods (if that's an issue) should vanish automatically if you start "relaxing into them". On the flipside, what many seem to do is "tense up" and try to counteract negative moods by e.g. smiling, which creates this eerie and unnatural kind of happiness.

Allowing negative moods/facial expressions to exist is a balancing act though. If your particular environment isn't accepting of sad facial expressions or you feel ashamed about it, then you have to find something in between, where you don't try to be overtly happy, but restrain negative moods so they're not projected onto others through facial expressions, comments etc.