There are two axes for food, or technically almost everything:
| | Healthy | |
| Yucky | + | Yummy |
| | Unhealthy | |
If something is in the lower left quadrant, you won't want to eat it and no one will want you to eat it. If something is on the right side, you'll want to eat it whether or not someone else wants you to eat it, so no one is going to come out with the "it's good for you" justification/exhortation. Only in the upper left section — yucky but healthy — are you likely to hear that. So it makes perfect sense that if it tastes bad, and a parent (say) wants you to eat it, it's good for you.
It should also be noted that taste is subjective, while healthfulness is notnote though it is subject to a certain degree of individual variation and continual research on the topic. So if a person doesn't like something, they don't like it, and arguing with them is unlikely to change that. "It's good for you" is much easier to demonstrate.
Evolution
There is actually a scientific reason that "healthy" foods are less flavorful in general. Back during our hunter-gatherer days, food was much more scarce than it is today, and so food that was high in fat and sugar (and thus loaded with energy) would be more sought out and thus more pleasing to the taste buds. Salt has been prized for centuries (remember the Bible quote "you are the salt of the earth"?), as it can be used to cure and preserve meat for long journeys, not to mention its role as an important electrolyte. Unfortunately, like the appendix, this taste for sweet and fatty foods still lives on in our genetic structure. Couple that with our less active lifestyle brought on by technological advances in agriculture, we end up consuming more energy and burning less. Thus, foods lower in fat, salt, sugar and calories are dubbed more "healthy" than our more decadent choices.
Furthermore, this same genetic hard-coding also explains why children in particular hate vegetables: we're sensitive to, and dislike, bitterness because a lot of things in nature which are bitter are also toxic; having an aversion to bitterness was a survival mechanism. As we age and our taste buds dull, bitterness bothers us less, so we come to appreciate things like coffee, tea and vegetables.
Myth
Sometimes, though, people are simply wrong. For example, cod liver oil is incredibly foul and really
not all that good for you. (It is, in fact, somewhat toxic if you eat too much.) You can get the vitamins in it without having to choke down the traditional liquid form. However, the idea that anything that bad must be good for you is, for some people, really potent, meaning utter crap (like some
herbal "medicine") can endure long after we've either discovered an easier way of doing the same thing, or found that it wasn't worth doing in the first place.
