It was supposed to be an 'inside joke' which only Mookie knew at first, then shared with Sam. This was because Mookie was the only one (other than Sam, of course) who knew that Sam had stolen (or borrowed, if yo prefer, since he returned it) the salt shaker.
: )

LOL. It's not that he likes salt much. The first time he just wants her mom to notice about the salt shaker not being there and he wants to talk about "someone's visit" last night. The second time during lunch he asks Sam for the salt as a complicit wink, because that was their secret.

Is it possible to have a problem where the body has a lack of salt? I think I heard about it once.