Similarly, if you eat something sweet after tasting a little salt, it will taste even sweeter.
When salt is poured over watermelon, the salty salt makes the watermelon taste sweeter. Therefore, a little salt on a watermelon makes it taste better. This is called the "taste contrast effect.
However, if too much salt is poured on the watermelon, it will only make the watermelon taste salty. The amount of salt that is sweetest is determined by the amount of salt you put on the watermelon.