It's worth remembering that most people who work in customer service have been around the block so many times they hate the people they have to deal with by default. Most folks I know who work in retail or the hospitality industry will openly state they hate you just for walking in the door.

Professionalism be damned, whether you're a disgruntled barista or a disgruntled anaesthesiologist, everybody needs to be able to vent by ripping into their customers sometimes.