We need sodium to function, but too much is deadly. Luckily our bodies know when we're in danger:
[W]hile mice like the taste of salt, past a certain concentration they find it disgusting. The same is true of us. This appears to be a biological mechanism to keep us from overdosing.
"With sweet, in general, the system is geared to finding it—if you detect sweet, you want to eat it, because this is energy and you need it. With bitter, you’re programmed to avoid it. There is no fine balance of ‘eat it now’ versus ‘don’t eat it’ at another instance," says Jayaram Chandrashekar, a senior scientist in the Zuker lab at Columbia. Although humans may develop preferences for a certain amount of sweetness or nurture a love of bitter coffee, on a fundamental level salt is the only taste that has this kind of switch.
The Dish 