Why do people believe in superstitions?

Why do people believe in things that are basically irrational, not even based on any experience or reason? Why would two persons shaking hands across the gate bring on themselves ill luck? Why will the bride cry all her married life if it rains on the wedding? Why will you have good luck through the year if you find money and keep? Why will two persons quarrel if they, while walking, divide so as to pass an obstruction one on one side and one on the other? Why would the nose itching foretell company: If on the right side, it means a man; if on the left, a woman? etc,etc,etc.