Why only large cities? I'm not really a city girl, I prefer smaller places where I can get to the mountains within an hour of driving if I want to (I live in Las Vegas, the population for the whole area is under 2 million).
Area population here is 9 million in Chicago metro (not my stat, from skyscapers.com). I've lived in smaller cities and I've found that they don't have as much to do. I'm used to having lots of neighborhoods and a
ton of things to do. The shopping here is a lot better than most cities. I like having different ethnicities, and tons of restaurants to choose from. I like having 2 baseball teams in my hometown

I like the constant hussle and bussle, even if the cab drives do almost kill me. I like the extra cultural benefits of having several major universities in town.
Thats why I like big cities.
I have to admit mountains, especially skiing, sounds fun if I want nature I can always get away to say Wisconsin or Michigan to get into the backwoods. Sometimes its dangerous here, but honestly overall I'm not even too scared of the bad parts of town. I wouldn't trade it for a small town, or a small city.