Most Helpful Guy
Smiling on a girl is a 30 foot trait, meaning it's like nice hair, pretty face & good body you don't have to know anything about the girl but from 30 feet away you can be attracted. So if smiling is the easiest of the 4 to control and a girl doesn't seem lighthearted or smile (not stupid smiling for no reason but not so stressed and gloomy) then in guy logic there is easier some issues or baggage and subconsciously it comes off as unattractive.
Smiling in a real and neutral way is very easy to do and I have been a big commenter on this and believe in it highly. Something happened in the last 10 years where every girl is acting like they have the weight of the world on their shoulders. Be it true or not the easiest way to get a date or been seen as attractive is to come off as responsible and a hard worker but a bit above the fray. Someone who sees the invisible rainbows that exist in every shaft of light.
Do that and you will grab every guys imagination or interest regardless of other factors.