Creation of Different Dialects

What factors contribute to the creation of different dialects? At what point is something an “accent,” a “dialect,” or a separate language? Why is English considered a “Germanic” language? Could you make a case for it to be considered part of a different language family? Why/why not?  Based on the increasingly diverse population of the United States and the past history of the English language, how might the English language change in the coming generations? What factors may influence these changes?