How and why does a person's accent change after moving to a different geographical area?

When I relocated from New York City to Los Angeles, someone at my New job after talking with him asked me, what country are you from? It was not long though before I began sounding like a Californian. The change takes place very subtly over a period of time. I have lived in many places here in the states and now have a well rounded accent. One thing you may become aware of is that when you become excited or are having talkative fun, your original accent peaks out and shows you it can and will come out when you become animated.