What's your accent?

I remember talking to one of my friends one day, and she made a comment about how she could tell that I was from the South. I was a little surprised, because I never really thought I had a Southern accent. So I began paying more attention to how everyone spoke around me. I have a cousin from Macon, Ga who I've always made fun of because of his strong Southern accent. But his sister barely has one. They are both around the same people half the time, so why does one person have an accent, while the other doesn't?

At the same time, certain parts of the South have people that are known for their Southern accents. But there are several areas where people don't have them. Why are there some parts of the south where people have accents, and other parts that don't?