My opinion about fast food is very simple, junk food is the worst way to feed people especially children. The problem about fast food is that is cheap, fast and accessible. You can find a fast food "restaurant" on every corner, also you can eat quickly inside the restaurant, walking or maybe you can also order inside your own car. This is a crazy idea but it works because people are lazy or busy and they prefer to eat inside the car than walk a little bit to the restaurant.
But the most important problem is that junk food is very afforable and the companies take advantage of that. A burger is cheaper than a salad and that is unbelievable. The craziest thing is when you need to choose between a salad or a burger, the burger is cheaper and it cames with fries, coke and toys.
In Spain we have just started with the obesity problem. The kids are eating more fast food than mediterranean food and the obesity in kids has increased a lot. It is important to stop that and we should came back to our original and regional food instead of the foreign junk food companies.