Why do so many service industry people act like jerks?

I went to the post office today to mail a package. I asked for the least expensive way to mail it that I could also have a tracking number. The guy actually sighed as if I was a bother! Then quoted me the most expensive shipping they have. “Overnight priority with tracking and 50.00 insurance will be $12.00.” No, I said the cheapest way, and he sighs again. I was the only customer in the post office.
I seem to come upon this attitude everywhere, although I usually walk up to counters with a smile and a hello.
Do you come across this a lot? How do you react?