Please, Thank You, & When to Hush
First of all, why do I call it an art? Please don't take this as my definitive definition of "art," but courtesy shares something with the arts as a whole because it is a communication between two people, which is made manifest by some sort of universal (or at least wide-spread) sign, and it contains an element of effortless grace (ask the perfect Renaissance man/woman about the cultivation of that skill.) Like any social or fine art, courtesy is something that requires observation of the masters, practice, practice, general adaptation of the basics as a matter of habit, and then more practice in pursuit of perfection.
Why is this art being forgotten? It's hard to pinpoint a particular "rudeness virus" but it probably has something to do with our culture's love of laziness (eg: going out of one's way to hold a door for an elderly person), lack of education (eg: it's easier for a parent to just let the kid talk with their mouth full and then leave the table without thanking the cook than to insist yet again on these rudimentary manners), and loss of respect for others (if we're all so darn equal anyways why should I recognize the presence of a lady or older person with any particular attention to my behavior).
Just yesterday, I stopped by the bank during my lunch break, only to find myself at the end of a fairly long line of people. Those around me we're cursing the line (only one was actually talking to another person on a cell--the others were just spouting), blaspheming God for no particular reason, and scowling at each other. If it wasn't for the obvious age difference, I'd think I was in a line of immature 4 year olds waiting for the icecream truck. I don't think these poor people had ever heard of self-restraint. In sharp contrast, the older man at the teller counter was all politeness and very helpful. Yes, he even smiled (so few bother to do so in public these days). When I left, he wished me a pleasant rest of the day in a cheery voice that couldn't have arisen merely from a company mandate to make customers happy.
Perhaps courtesy is not completely doomed after all, but please do remember to smile.
It makes such a difference.