Imagine a delicious plate of pad thai, fresh and ready to eat. You can smell the peanuts and chili powder. You can see the steam rising as you prepare yourself for the first bite.
If you're like me, then this thought probably elicited at least somewhat of Pavlovian response. You probably have at least a little bit more saliva at the ready for the pad thai that's not there. No worries, we can just swallow the extra saliva and move on with our day. Nothing weird about that last bit, right?
Now instead imagine spitting that extra saliva into a clean cup. And imagine swallowing the saliva from that cup. Yeah... Bleh.
What is it about these two acts that elicits such a different response from our brain? By all reason we should be pretty much indifferent between these two acts. Yet despite all our enlightened faculties, some deep primal disgust cannot be overriden. How strange our neurochemistry is, and how poorly we understand it. Next time you feel disgust or revulsion, pause to think about; it's likely you do far more disgusting things on a daily basis.