oh you know what? i also think a factor of how rude people can be goes hand in hand with the "only-child syndrome" thing.
doug and i bump heads on this one a lot. i grew up an only child, so i tend not to think about anyone else's needs over my own wants (i'm working on this! i swear!) but doug grew up as the baby of 5, and his whole gosh darn family is so helpful and kind to anyone they meet (for the most part haha). he is just more worried about others, but i have never had the experience of needing to. or at least i didnt realize it until i met doug and he called me out on it. can you believe i've pretty much only ever dated only child guys too, so they were just the same as me, i never realized it!
but on the other hand, sometimes he just doesnt get it that its hard to learn to be "different" from how i've always been when no one in my entire life has ever told me about this!
anyways, i digress, BUT i think it could be part of the whole rudeness thing, people may just not know that what they are doing is rude. they are just selfish, but not because they are mean people on purpose, but because its just how they were brought up. people just dont stop to think what the result will be, they just act/react how they want.