Why are things deemed as nasty as soon as they are deemed to be unwanted?

Asked by Nimis (13255) October 9th, 2012 from iPhone

That other question about nasty leftovers got me thinking about this again.

Why does something that you were just eating suddenly become nasty once you’ve had your fill? Unless you’ve had it hanging out for awhile, what exactly is nasty about leftover soup?

Or hair from a haircut. Not even other people’s hair. I’ve seen people grossed out by their own hair. It was just on your head a second ago!

I’m not asking to rant. i genuinely don’t understand this. Admittedly, I can be a bit dense about such things. Any explanation beyond It just is! would be much appreciated. Thanks!