Jeneta: Increasingly, I've noticed that when a salesperson thanks a customer for making a purchase, the customer also says "Thank you" instead of saying "You're welcome."
Given that ...
We say "you're welcome" when a friend says "thank you for doing me that favor".
Why is it that ...
We say "thank you" when a cashier says "thank you for your purchase"?
Evaluate / Goal
We need a way to differentiate these two situations so that it makes sense why in a store we reciprocate a "thank you" with our own "thank you", whereas with friends we reciprocate a "thank you" with a "you're welcome".
While it's usually dangerous to predict answers to Paradox, this one seems fairly normal to understand from normal life.
With a friend for whom we've done a favor, they've gotten benefit but we haven't (other than the fuzzy feeling of doing something nice for a friend).
But when you make a purchase at a store, the cashier has benefited by getting your money and you have benefited by getting the product you just purchased. So maybe we say "thank you" because we appreciate the product we're receiving, or maybe the salesperson helped us find the right product or did something else to benefit us.