If you got to really know them, and they didn't smile, I would not trust them either. But some people really need to get to know you before they feel comfortable around them.
Maybe some people don't smile because they need some dental work.
or maybe they don't smile because they just don't like the person...
I'm sorry but i don't think your putting up what i'm putting down, i'm not talking about people i know and don't know, people that i do want to know or people that i don't want to know, i'm talking about people i interact with through one way or another.
For example, i go to the gym four or five times in a week, three days a week there is a British girl behind the desk, the other two days the girl is Polish. Usually you walk in and have to have your card swipped through a machine, then its 'thank you...........' maybe a bit of conversation if i or them are up to it. When the British girl says 'thank you' it is usually accompanied with a smile, however the Polish girl, never a smile.
From my experience its the same when i go for a meal to a specific place i go too, the waitresses there are Brits, Poles, Africans etc etc, the Poles never smile when they serve you. While the Brits, Africans sometimes indulge in a little conversation if you/them feel comfortable, what i'm trying to say is, while the Poles are very good at hospitality, they do operate with a certain 'robotic' style. THis is something i'm not really looking for when i'm out eating a meal with friends, a place that is meant to be comfortable, warm, friendly etc etc with robotic staff??? lol it does not add up for me, so i rarely tip them well.
I could give many many examples, even of people on the street who just need something, time, light, even a bit of money if they have not got enough for the bus (yes its happened) Still no smile or thank you, apart from anything this is rude (maybe a cultural difference i don't know)
I've never had a problem with my Polish friends, i do say now and again 'come on cheer up' or 'smile'. Many of them are cheery, smiley and friendly but i do find sometimes that smiling is difficult to come by when your eastern european. I know i'm generalising a little but how else can i put it???.................