I think that's not just me, it's the whole idea of the pan/pani thing. In Poland, when you let somebody call you by your first name, you allow the person into the inner circle, so to say - grant them a whole package of rights and priviledges which you wouldn't like to grant to just anybody.
In the English-speaking countries the clues are different, as you call everybody by their first name, you need different signals to discern between the outer and inner circles.
Simple projecting signals from one culture onto another often leads to confusion and misunderstandings.
Now, a smile for you is like pan/pani for us - one is supposed to show politeness and approachability, the other - respect, bot both are just social conventions and can be equally phoney and if you take them on face value, you may end up bitterly disappointed.
OK, to make a long story short: a smile for Poles is not so much the matter of a social convention. Therefore in Poland you see less smiles, but if you see one, the chance is bigger that it's genuine.
It's all a matter of social codes ;)