People tell me I don't smile much, and it's for two reasons: 1. As an aspie I find I often don't know how NTs are reading my face so I try and keep a straight one as much as possible and 2. I always felt like an open smile directly at someone's face is an intimate act that sends a direct message that can be: I like you, you make me happy, I want to be friends, I find you attractive, etc. so I never do unless it's with family or good friends. NTs don't see it that way at all: they view smiling as impersonal and meant to be pleasing and polite. On a side note, I find that aspie girls actually smile a lot, but don't like being smiled at. I knew one who laughed and giggled around me all the time, but when I shook her hand and said "Nice to meet you" with a smile, she turned red. Very confusing.
As you can imagine this has caused me some confusion when around neurotypical women. I have a very hard time understanding when a smile means actual friendliness or attraction and when it's just a polite, public smile: the smile of a waitress. Even worse, I have no idea when a smile means a cover for discomfort. NT have had to explain this to me over the years, and I get it, but I still have a hard time identifying it.