Its not the smile itself but the position from which you show it.
You introduce yourself to someone you smile and shake his/her hand is fine as long as the body language is ok.
You smile after someone made a lame joke or worse made a joke on you, only to look "friendly" and its not ok.
Some woman is showing you her kid and you smile to look "nice" and despite what you think its not ok, nature didnt want grown man to like someone else kids, you do the rest.
Regarding the smile it may work to create emphaty among stranger and show lack of bad intentions (or fake such lack), just think about car dealer which are probably the most smiling individuals, the more they smile the more they are pushing it in your ass during the agreement.
So again I confirm that it depends from which position you smile, but even more important than the smile is laughting, someone which does it too much is either stupid or submissive, or insecure and trying his best to create some connection with someone.
Smiles are precious things and those who abuse of them either have personal problems or bad intentions toward you.
The ancient romans were used to say that "laugh is plentyful in the mouth of fools".