Thank You Obligations

The other day at the grocery store, I let a woman in front of me in line because she only had one item and I had a full cart. She said, "Thank you", and then after a few seconds turned around and said, "You know, that was a bit rude". "What do you mean? I just let you in front of me!", I replied. She said I was rude because I did not return her "thank you" with a "you're welcome".

This is ridiculous.

The "you're welcome" response should be an understood part of this sort of interaction. By allowing someone access to a good or service is an indication that they are already "welcome" to it. A person shouldn't be obligated to say anything after someone says, "thank you" to them. They already tendered a favor. To have to retort to the expected response of "thank you" is a bit much.