@thiet sun,
Yes, I do agree that in some big cities the quality of life is decreasing. When I refer to quality of life, I am coming at it from a different angle. Yes good paying jobs, lower unemployment rate, and pollution free environment are very important to quality of life. I would ask the question in a different manner. I would ask: What things would make you want to live in any particular big city? I would want to live in a big city that doesn't have a whole lot of heavy traffic congestion on the streets and highways. I would want to live in a big city where I don't have to drive far to get away from the city to enjoy nature such as beaches, mountains , lakes , rivers, waterfalls, animal life, forestry, caverns, birds, flowers, etc. I would also want to live in a big city that offers a large variety of affordable inexpensive restaurants. I would want to live in a big city that offers an assortment of free or inexpensive live entertainment in small intimate settings. I would want to live in a big city that I can find something to do 24 hours a day. I would want to live in a big city that has a low crime rate. I would want to live in a big city where I can get inexpensive quality medical care. I would want to live in a city where I could get inexpensive quality schooling. I would want to live in big city that is spacious, clean, open, and beautiful. Those are some of the things I would be looking for when it comes to quality of life.