One
reason is because salt was once valuable and difficult to obtain. According to
an old Norwegian superstition, a person is doomed to shed as many tears as it
takes to dissolve the spilled salt. Another reason is the belief that spilled
salt refers to the devil. In the German superstition, whoever spills salt
engenders enmity: It is thought to be a direct act of the devil. And both the
French and the Americans toss salt over the left shoulder to hit the devil in the
eye. (The Book of Answers, by Barbara Berliner)