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xkcd #2932: Driving PSA
(imgs.xkcd.com)
Alt text:
This PSA brought to you by several would-be assassins who tried to wave me in front of speeding cars in the last month and who will have to try harder next time.
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.