A few commenters mentioned the food reward theory in previous posts. One thing that should be pointed out is that rewarding food is not going away anytime soon (barring regulation), and that simple logic would predict that it will continue to increase.
Competition between restaurants and food manufacturers is all about gaining customers and profits. If one company can change their recipes and gain business, then other companies will surely try to respond with an enhanced recipe of their own.
This sets up a game theory situation like we talk about in economics. Restaurants and food manufacturers effectively engage in an endless arms race of trying to create more rewarding food. Of course, this may not be beneficial for consumers as this food can be semi-addicting.
In fact, a new paper by Andrea Garber and Robert Lustig asks that question in the paper's title, "Is Fast Food Addictive?" I would say that even if fast food isn't addictive yet, it most likely will end up that way as restaurants look to keep pace.
Economic logic strongly predicts this will be the outcome.