What ever happened to manners?

I went to Subway for lunch today. When I paid for my meal ($7.50), I gave the cashier $8.00. There is an automatic change machine there, so out rolled two quarters, followed by a dime. I looked at the receipt to be sure I wasn’t imagining things. Then I said to the cashier, “Your machine gave me ten cents too much.”

She frowned at me, took the dime, and threw it into the tip jar.

Now why couldn’t she have at least said, “Thank you”?