I'll admit to throwing salt over the left shoulder if I should spill any, but I do not consider myself superstitious beyond that. As someone mentioned in another post it can be convention or habit to do certain things.

I do want to mention the black cat idea though. It is said to be bad luck is if a black cat crosses your path. However, I have also read that a black cat is the luckiest to have in your home. White cats in the home are said to be sickly and bring bad luck and illness.

From my studies and interest in both history and human behavior, it does seem that we have an innate tendency to look for patterns. It could be simple biology. If we learn that those long, thin creatures slithering on the ground are usually poisonous, then it is easier to survive and assume all snakes could hurt us, rather than learn each and every variety and whether they are poisonous or not.

In olden days, when people had practically no understanding of scientific principles, they saw what usually happened and created rules from that. Some were more accurate than others. But I can see how it easy it would be at the time to blame contagious illness on what they saw as a curse or evil person. Think about an ancient village where a plague breaks out. Not understanding viruses or bacteria, someone might see that someone became ill after being near one person and interpret that as a curse spreading through the village.

Or if a village is struck by a drought or flood or other natural disaster, it seems obvious to blame the gods when one does have the knowledge of meteorology and such.