Likewise, I've been honoured to be the recipient of acts of kindness and always try to "pay it forward."
On a recent flight through DEN that was overcrowded and WAAAY-overdelayed, I began chatting with some of the fellow passengers. One young lady was on her way to Korea and had already missed the connection out of Seattle (one flight a day). Knowing that she was in for an even longer day than I, I told her to give me her boarding pass. After she reluctantly handed it over, I anted up some upgrade certs and got her upgraded for the flight. While it certainly didn't fix her problems, it made them just a little easier to swallow, I hope.
On another note, I always try my best to be polite and even helpful to the FAs. Especially when there are "difficult" passengers on some of the flights. It's not an easy job!
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"What do you mean you didn't get miles for that?!"