My guess is that the USPS work rules are as onerous or more so than any of their competitors. The idea that postal workers may have to “work up a sweat” during the course of their duties should be a good thing. As for your mail carrier being friendly, any group has a few good workers. I think if you polled people you might be surprised that most folks would not share your enthusiasm for their letter carriers.
Rasmussen ran a poll a few years ago which showed the public had a generally favorable opinion of the postal service. I don't doubt a bit your bad experiences, but as a whole, I'd say most postal employees, particularly the long-term career employees, are generally a decent lot.
http://www.rasmussenreports.com/public_content/lifestyle/general_lifestyle/us_postal_service_gets_favorable_reviews_despite_higher_price_of_stamps