- Scat (poop fetish, not singing really fast nonsense)
- Killing another human
But treating this theory of why as an explanation is folly since it doesn't address the what. We accept this answer because we're used to other, similar answers where, when the why is identified, the what just falls into place.
Take, for example, a "Bridge Out" sign. Why is the sign there? Obviously because the bridge is out. We neglect to mention that the sign is actually there because someone went and put it there. In this case, it doesn't matter because that much is obvious...but not everything works like that.
The action is not always readily explained by the reaction.
So there must be something, some part of the human mind, that makes these things unacceptable. Or something. But it goes further. What about these "facts of life":
Luck...I got nothing on that one.