I think you get used to it and you learn to crave it. When I was a graduate student, poor and hurried, I ate fast food regularly--McDonald's, Pizza Hut, KFC, etc. My diet was terrible, but I was only a little chunky because I was also very active. In my late 20's I started to think about my health and I wanted to take up running more seriously, so I gradually cut out all the fast food, gradually lost my taste for it, and also lost about 30 pounds, from chunky to skinny. Now I feel disgusted if I try to eat anything as greasy as a standard fast-food hamburger or fries.
As others have noted, there are "fast food" places that are pretty healthy. Subway has some gloppy greasy stuff but a lot of their sandwiches are really pretty healthy and tasty. Subway is my go-to place when I'm on the road and need some food fast.