Why is spilling salt considered bad luck?

One reason spilling salt is considered bad luck 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.