I mean that some people have bad manners, and you know... like you said, they burp once in a while and so, but they don't know any better because they haven't been exposed much to good manners. Other people have bad manners even though they know exactly what good manners are, they are just making sure to show the world that they have bad manners for some damn reason... those are the ones that get to me a tad.
I cannot stand bad mannered people.... When they say stuff like "give me that".........and never followed with a please or thanks..... Uugghhhh..... When I was in NYC I wanted to punch most of the taxi drivers there.....so RUDE.... I said to one "have you never heard of the word thanks".....he looked at me like I was an alien... haha!!
I think it's less about being molded to have habits at a young age, and more about consciously choosing to be courteous (rather, taking into account someone else's wellbeing and comfort along with your own needs). I'm thinking intergenerational manners and social etiquette would be an interesting study to look into, as in, how my parents cannot seem to understand why I won't answer a cell phone while in a restaurant.