Consuming fats, sugars and salt gives the brain an chemical rush of pleasure, lighting up the reward centers of the brain like a Christmas tree. Over time though the high and feel good you get from this diminishes and you have to eat more and more and more to get the same feeling. Fats lack self control and seek this high every chance they get it. It's no different than the same thing any person gets from eating  food except they lack self control and in their empty lives chase it over all other things. This is why they always go  for the beetus juice, put mayo on fries, a ton of salt or ketchup etc etc. They are feeding their addiction and lack of self control not themselves.