there are girls who don't smile. they are often shunned. a smile has many functions, it is a very welcoming gesture, one that seeks social contact, tries to please or disarm, or soothe among others. a warm smile is likely to make people happy, feel at ease, and it can cover up a lot of imperfections. this is true for guys as well. they tend to smile less often, but when they do, it makes me feel really good. a smile can be contagious, so if you smile even when you're insecure, people will easily think everything's in perfect order, forget to notice any broad shoulders or angular features, and just smile back at you thinking you're just the person you present yourself as.
i don't know if smiling is a typical feminine trait. many women do it to be nice, but an open smile can get her a lot of unwanted attention, whereas a more resigned one will just make people feel ok. and i think a fake smile is something many have learned to produce through social programming. girls are often expected to smile, so they do. at least that's what i do, i've perfected my fake smile, but unfortunately that makes me unable to tell people off convincingly. never got the opportunity to learn how to express negative feelings in a natural way, but i think that has more to do with my parents than the rest of society.