Why are people superstitious?

Dieter Harmening's book Superstitio categorizes superstitions in three categories of magic, divination and observances, he further divides observances category in 'signs' and 'time'. Time sub category constitutes temporal prognostics like observances of various days related like dog days, Egyptian days, year prognosis and lunaries, where as signs category constitutes signs like particular animal behaviors, like the call of birds or neighing of horses or sighting of comets, or dreams.