Humans are known for mirroring each other socially . When you see someone smile at you , naturally you will smile back . Well , most people would smile back unless they really are having a bad day or etc
They're more likely to, but it depends on a lot of factors. I smile at everyone and am very friendly IRL (I do not suffer from RBF) but almost everyone responds negatively or indifferently to me. Typically, people look disgusted, annoyed, etc., and basically like they can't wait to get rid of me. I can occasionally overcome this reaction if I get to know someone, but most people have no interest in getting to know me.
This has always been a problem for me (I was very unpopular in school) and has only gotten worse for me as I've aged. If people are "having a bad day" then almost everyone is having a bad day almost all of the time. And it doesn't explain why the same people are nice to other people they talk to before or after me.
Being friendly and smiling is not a guarantee that people will treat you nicely. People respond to a number of different cues, like your attractiveness, gender, ethnicity, social status, how 'normal' you are, etc. People tend to reserve their smiles for people who meet their various thresholds.
I don't particularly care that people don't like me (which, I agree, is good) but even if you don't care it still has a negative impact on your life. It's sort of like having to live with a bad smell everywhere you go. Being sneered at isn't pleasant even if you don't care what a person's opinion of you is, because of the way human brains are wired to respond to social cues (the mirroring thing). Also, people who don't like you tend not to give you very good service, which is a problem in itself.