I don't really agree with that. I know people have bad days, but when you've got good manners, it's almost a reflex to be at least courteous -- e.g. to say thanks if someone holds a door open for you -- regardless of how you're feeling at the time.I don't really stress about it and let it fly over my head mostly. There's so many reasons for impoliteness, they could be really busy, running late, having a bad day, stressed, anxious, shy, grieving (esp older people, we tend to underestimate how much loss they've experienced by that age, no wonder the poor buggers are miserable) it doesn't always mean people are being deliberately rude to you or that they're like it all the time. Plus it makes life more interesting, it would feel a bit stepford housewives if we were all perfectly good mannered at all times
I remember just before the first Father's Day without my Dad I unexpectedly got emotional in the supermarket seeing all the Father's Day cards and it suddenly dawning on me that I'd never need to buy one ever again (thankfully my face mask hid most of my tears!) I still said hello and thank you to the person on the checkout though, because it's what you do. (Or should do anyway.)