Not true, Sorry, spent pleanty of time in densely populated parts of the North and South as well as sparesely populated parts of both...
I can tell you this much UNEQUIVACABLY... while it is true the level of courtesy does go down as you get closer to densely populated areas, whether it be the south or the north, there is ABSOLUTELY no question that the general level of courtesy is far far higher in the south than the north.
Yes it is unfair to compare rural Alabama to NYC, but Atlanta GA to NYC and in the courtesy dept hotlanta wins... Small town south vs small town north as well, general level of courtesy the south wins, hands down EVERY time.
I’m not condemning all northerners as clods, but apples to apples, courtesy generally is more practiced and respected in the south.
Friendliest cities, IMHO are Louisville, Dallas, Fort Worth, San Antonio, and Nashville. Chicago is probably the friendliest city in the world with a population over 1MM.