That is indeed very surprising to hear.
Of course, my main impression of America and Americans comes through media and pop culture, and there seems to be a definite tendency to portray American culture as extroverted and group-oriented. Americans i have met here and when travelling have been super chatty and outgoing, too, so i just assumed most Americans were naturally extroverted.
I've heard the South has a reputation for being especially friendly and courteous, the famous 'Southern charm' and all that - why do you think there are these regional differences?