1. Nutritional needs depend on age, time, body, activities, etc.
2. The desire for tasty food often greatly out-weights the desire for nutritional food.
3. Evolutionarily, we were designed to want more than what we need.
#2 might be a little trickier, although if it's sold how it's shown in the show (literally as human dog food in giant bags) you could easily include a dozen flavor packets. If the food on its own was fairly flavorless, preferably with a meat-like quality to mix well with different flavor packs.
#3 is probably the trickiest. The solution is probably just like any food, moderation. However, when people receive adequate nutrition this behavior does tend to decrease, but evolution is a harsh mistress and like most animals we've evolved to build up fat reserves to try and survive.
Honestly #1 and #2 can easily be solved by standard market practices in both dog food and human pre-made meals. #3 is the stickler and aside from moderation I don't know what could help, except maybe a big sticker saying 'THREE MEALS IS ALL YOU NEED' might subconsciously stop a few of the gullible people who respond easily to suggestion (read: the people who keep telemarketers rich).