Why Everything Good Is Bad for You
Imagine an animal that requires some substance, but that substance is scarce. Say, a mountain goat that needs salt, but lives in mountains where there’s hardly any around, unlike the seashore. In such an environment, it’ll need all the salt it can get, and natural selection will favor those goats that find salt tasty, since they’ll seek it out.
Obviously, it’s possible to have too much salt in one’s body, so ideally there ought to be a biological function that switches off this craving for salt. But in practice, if salt is that scarce, there’s no such thing as too much salt, so the mechanism that switches off salt’s tastiness can be horribly miscalibrated and still not be selected against.