It is rude to be stingy with tips.
John is having lunch at a restaurant and the service was poor.
John wants to leave a tip after paying.
John decides to leave a good tip because he noticed how overworked the waitress was and felt bad for her.
John leaves some change as a tip in order to indicate his dissatisfaction with the service.
The waitress feels like she didn't deserve his generosity and works even harder to keep the rest of her tables happy to overcome her guilt.
The waitress becomes sad at the bad tip, causing her to mope and the rest of her tables to be dissatisfied as well.