Smiling is a valuable skill, and well worth learning.
I used to never like my face in photos, up until age 16. Someone told me to just relax my face, rather than telling me to smile, and the photo turned out great. Relaxing my face felt great, and so I had a small natural smile. YMMV, but it looked natural, and made my face look better than a big smile.
Years later, after I realized that my default face settings diminished my effectiveness, I started getting in the habit of using that trick more often, and eventually that little smile became my default face setting.
The Facial Feedback Hypothesis is worth knowing. Pick a source which suits you, there's a lot written about it.
Instead of getting annoyed at people for doing what they aren't going to stop doing, you could learn a valuable new skill. Philosophically, I want to live in a more beautiful world, so I make myself as attractive as possible in order to contribute to the general sense of a more beautiful world. Like the Style Golden Rule, or something.
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"I find that the best way [to increase self-confidence] is to lie to yourself about who you are, what you've done, and where you're going." - Richard Ayoade