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xkcd #2932: Driving PSA
(imgs.xkcd.com)
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It reveals how they weren't actually being polite. They were doing it for themselves, not you, and you denied them their "good deed" dopamine.
You're right of course, but in a broader sense there is literally no action that anyone takes that is altruistic. We only do things that benefit us or, rarely, our group as a whole.