Some superstitions are founded on common sense. The "walking under a ladder" because there's usually somebody at the top of the ladder dripping paint (or whatever).!
Black cats are supposed to be witches reincarnated (from medieval lore - see how many paintings from that time have witches with black cats). Hence, they are bad luck if they stop and look at you, but good luck if they "cross your path" completely ignoring you.
13 is supposed to be unlucky because of the number at the Last Supper.