We're trying to have a society here.
You know those Marilyn Monroe quote memes on Facebook? The ones that say, "If you can't handle me at my worst, then you don't deserve me at my best." Whenever I see that, I immediately know that the person posting it is 99 percent worst, and 1 percent best.
Why? Because they're flat-out stating that they take their sh*tty attitudes out on other people. They're indirectly saying they don't understand how their behavior effects everyone around them.
If another human being can't handle you at your worst, then maybe, just maybe, you need to look in the mirror. Outside of social media (yes, it exists), you see people everywhere blaming their terrible attitude and manners on external factors.
But here's the honest truth: If you're only polite when it's easy, then you're not a polite person. Anyone can be well-mannered and pleasant to deal with when they're in a good mood. But if you're rude when it's easy to be rude, then that's what you are.
Unless you're a sociopath, no one is being a total d*ck to other people when they're filled with joy. The concept of politeness is only a thing when it clashes with how you'd want to act.
It means suppressing whatever ruined your day because it's not your waiter's fault. He didn't pass you over for your raise, and he definitely didn't overcook your steak.
So many traits can be judged this way. You have to see how the person handles things under adversity, not ideal conditions. And consider this a sub-PSA: When it comes to interactions with strangers in public, no one gives a sh*t how you're feeling.
So, the fact that you're in a terrible mood gains you no sympathy for your rude behavior.