Why is it considered bad

Why is it considered bad luck to spill salt?
"We take salt for granted now, but it was once considered very valuable (after all, without it your food is terribly bland). In ancient Rome, the seasoning was so precious that soldiers were given special allowances for salt rations, called "salarium." That's how we got our modern-day word - "salary." Since salt was so valuable, it was bad luck to spill it. If you did so, you were supposed to throw a pinch of salt over your left shoulder to nullify the bad luck. (More trivia - In Leonardo Da Vinci's famous painting, The Last Supper, you can see that Judas has spilled the table salt. It's the artist's way of foreshadowing the tragedy that is about to follow."
- Thanks, Adam!