A very revealing, albeit disheartening, truth. The food industry intentionally adds ingredients into foods to create food addicts and that shut down that part of your brain that says just have one.
The book is called Beating the Food Giants by Paul Stitt. Even though this was written in the 80's it is more true now than then. There is also massive amounts of money spent by food companies on the psychology of food.
Vanilla is the most popular flavor because it is formulated to be an almost exact replica of breast milk/formula. Fast food is so great because it has a "pre-chewed" texture like baby foods, thus creating the association with comfort and love.
You can find Mr. Stitt's book if you search the title and add PDF after or PM me.