If somebody will smile without the reason when talkinkg to me then I will think that he considering me funny, that he believe that I am a clown etc. I will think that he is not considering me serious. The best thing I can do in such a case is stop talking with such impolite person.

Sometimes sellers cheat me, making me to pay more than I should. I noticed that they smiled all times they cheat me! They use smile to distract my attention.

Russian proverb:
«Смех без причины — признак дурачины»
It means «if somebody smiling without the reason then he is mentally ill» (in mental houses you can often see people walking around and smiling continuously).
Another one:
«Улыбайтесь! Это всех раздражает!»
It means «Smile! That annoy everyone!» (If you smiling while it is hard time for the society then you are hard-hearted and ignore common hardships).

Also it is impossible to work and smile at the same time. So, if someone is smiling at the workplace, then he is not working at the moment and doing something irrelevant. Then, if the boss will come and see that somebody is smiling, then he will know that the woker is not working (maybe he reading funny stories at the Internet instead).