Why is someone else's mess (flatulence, boogers, poop, dandruff flakes, etc) so disgusting to us, but not our own?

You know the drill. Someone in class is throwing boogers at random students. But your own boogers are not as gross to you.

Or, you have a bad case of Diahrettic Gurglebutt. The stench can clear a building, and you know its much worse than usual. But you can take it, because you ‘own’ the stench.

Dandruff flakes, snot, body odor, bad breath, hairs in the sink, even the mess you leave on your plate after dinner—may revolt others but to you yourself its not so disgusting even if you know it is.

There must be a scientific or psychological explanation for this. Maybe to protect us from toxicity of foreign things but our own bodies are safe to us?

One exception is vomit, which can be pretty disgusting even to ourselves.

Why is this?