What Makes A City? (cities, location, population)

[BostonJad]

One thing I always wondered about is how did the biggest cities come to be where they are? does geographical location have any factor, I understand that is it is strategic to lie on the coast as it facilitates international trade. Does that explain why NYC and LA are the biggest cities?

At the same time you have Boston, Miami, Seattle, San Diego and San Fran lying on the coast, how come their populations dont compare to the former two?

Of course, there are many border cities in between those, but none rose to the prominence of those mentioned. What plays a factor here, is it weather or anything geographical?

How about the cities around the Great Lakes like Chicago, Cleveland, Milwaukee, Buffalo, Detroit and even Toronto, did they happen to fall in a strategic point of the lakes?

I also notice that all great cities have a waterway flowing through them think Charles River for Boston and Potomac for DC. That is not always true as you can barely find any waterways for cities like Atlanta or Tucson.

But Atlanta is an interesting example, how come it came to be the great city of Georgia and not Savannah that lies on the coast?

Is there any political or cultural factors that come to play on why many people decide to settle in one particular location. I am curious to hear the many thoughts on this.