I came across this idea in
William Ian Miller's book
The Anatomy of Disgust. If you think about it, it's really true: things that would
disgust you in a stranger are somehow
tolerable in someone you care about.
For example, one normally surefire trigger of disgust is bodily fluid. Most people don't want anything to do with the saliva, snot, semen, blood or excrement of another human. Unless that person is someone you love. You wipe your kid's nose or change their diaper without a second thought. You kiss and exchange all sorts of bodily fluids with your lover. If an aging parent one day needs help going to the bathroom, most people wouldn't hesitate. But (unless you're a health care worker) think about doing any of these things for a stranger and your stomach turns.
Disgust is one of the first symptoms of a failing relationship. You know you're falling out of love when the hairs in the sink start making you sick. Disgust is a sure sign of alienation.