I had an issue with something on the hotels.com website. I contacted an agent who was mostly polite and actually resolved my issue. I said “Thanks so much for your assistance! Have a nice day!” Rather than leave it at that, the agent started asking me why I didn’t understand their policy (keep in mind the issue was already resolved!) and how come I didn’t read the terms of service more closely and warned me that they would never make an exception for me again.
…
what’s the point of going on the offensive after the issue is resolved? She could’ve just said, “You’re welcome! Please note this is a one time exception!” or even just “Goodbye!” instead of leaving a bad impression at the very end. 
So what was the issue?
my rewards nights were expiring due to ‘account inactivity’ but i actually had plenty of activity on my account, it just wasn’t showing up in the right place. i asked her to extend the expiration date so i don’t lose everything.
The Border Security industry tends to attract the most egregious assholes among us. Whoever isn’t qualified for a job there goes into Customer Service. That’s just my theory.
Some customer service agents are poor. Often Customer Service suffers from inadequate staffing and low pay. The bad reps should be fired and replaced with trained decently-paid representatives in numbers allowing for proper customer service.
I am not holding my breath.
Phone agents are often under continual review and HAVE to respond to everything a customer says according to scripts and the like. In thanking her, you might have said some phrase she was bound to answer in a certain way, no matter how obviously a simple “you’re welcome” might have done.
Phone operations and call centers are so obsessed with the numbers on their calls and used to dealing with stupid, unmotivated employees that many of them use shelf-fulls or rigid procedures that agents have to comply with or die.
None of this works to make CS calls a pleasant thing for anyone. Only in some cases, some times. Little wonder voice mail hell systems have become so popular, besides just their staffing reductions.
I’ve had two relatives who worked in customer service. One had a fairly decent job helping people with insurance claims on GE products. They did an okay job of training them, and even though he didn’t personally care for the work it was clear that the supervisors were attentive and retention wasn’t ad.
The other relative was very definitely at a McJob that paid their workers a pittance, treated them like garbage, and fired them at the first possible opportunity. Their turnover was extremely high and they put zero effort into keeping people around over a long term. It was a terrible place to work and she quit as soon as possible. To say that her co-workers were not exactly brain surgeons would be a terrific understatement.