It's the basest of survival instincts. If you didn't think that "your crap smells good", you would have no incentive to reproduce and pass on your "qualities" to your progeny.
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Why do some people think they are fat and others see them as thin ? If you tell them they are too thin, they might feel insulted, because they really think they are fat.
That would (almost) never happen in today's world. At least not in America. You can never be too thin or too rich!
