Let's say you're at a shindig of some sort - an office holiday party, a department function, the post-lecture refreshments, a gallery opening, a con, any place where there are a lot of other people, most of whom you don't know personally or well, and a lot of milling around.
In the course of this milling around, someone steps on your foot. Someone very much larger than you, wearing big, heavy shoes. And doesn't move off. You can't pull your foot out from under theirs. They're talking to other people and don't even notice through their thick soles and distraction, what they not only have done but are continuing to do.
You go "Ahem," and "Excuse me," and "Sorry, but you're standing on my foot," in a polite if urgent tone, but they can't hear you, partly because they're too tall, partly because of the party hubbub, and partly because they're not listening to you but instead the sound of their own voice and the appreciative remarks of their friends.
Your hands are full, so you cough louder, but it doesn't get any results. At this point, you can't even feel your toes any more. In desperation, you awkwardly shuffle your drink onto your plate and tap gently on their shoulder, but they don't notice it either. (Perhaps a couple of their friends have, though, and are discreetly trying to break into the monologue, but having no luck.)
Finally, in desperation, you raise your voice and shout "EXCUSE ME!" The foot-trapper turns their head to look over their shoulder, without moving their feet, and goes "What?!?" (Perhaps they brush their elbow against your arm, making your drink splash and scattering a cheese cube or three.)
"You're standing on my foot," you point out. "Can you get off it?"
"You're rude!" says the foot-squisher. "You shouldn't interrupt people in the middle of a conversation. Now I've lost my train of thought!" or maybe "It's rude to shout at people!" (Perhaps even, "Look what you did to my coat!")
And then they turn right back to the conversation they were having, without bothering to move.
When you point this out, again, they say smugly, "If you'd asked politely, I would have."
--I started working on this analogy* a while back, encountering an older post on the DAZ forums (reg req, but free & searchable) defending the use of the word "Oriental" as a neutral, purely-directional term even when applied to people who objected to it as loaded with lots of negative cultural baggage and on the wording for this post yesterday a.m. after having been involved IRL stuff (and staying far, FAR away from HP fandom because my copy of 7 is still waiting for me at the post office (I hope!)) and have been offline since then, with a brief jump-on yesterday evening to check my email and see if there were any emergencies. The analogy, and the next one, have been in the mental works for a couple weeks, since I tend to mull things over for a long time and formulate them slowly (except when I get on a tear and just fly nonstop off the cuff until I crash again.) The fact that it's coincidentally applicable (again!) is just proof of the need to have written it.
This analogy is based on several real-life incidents, btw. Some were as a small child, and some were as a small adult. The only thing changed is the ultimate refusal to move - the indignant and wounded tone of the foot-trapper when I finally gave up hoping they'd just step off, so I wouldn't have to endure the backlash from initiating a "selfish" confrontation, and got the nerve up to yank a sleeve hard enough or shout loud enough to be paid attention to, is something I remember all too well. ("You shouldn't have been standing right behind me!" "Why didn't you say something earlier?" "Yelling at grownups is rude!" - no matter my foot was bruised for many days thereafter.)
* To make the analogy quite complete, the bystanders should join in loud agreement as to how rude it was of you to shout, and how you should be the one apologizing, for having interrupted, for having splashed your drink on the speaker's outfit, &c &c ad infinitum ad nauseum.