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xkcd #3191: Superstition
(imgs.xkcd.com)
Title text:
It's important to teach yourself to feel responsible for random events, because with great responsibility comes great power. That's what my wise Uncle Ben told me right before he died; he might still be alive today if only I'd said rabbit rabbit that year!
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3191/
It's because we're just pattern recognition machines and a false positive is better than a false negative.
So if an act might result in a positive or avoid a negative, it's safer to just do it anyways. Giving people who get neurotic about it enough of an evolutionary advantage that it's still relatively normal human variation.
Maybe the environment changes to where that OCD behavior is really advantageous, then the rabbit rabbits will inherit the earth.