The mathematical theory of surprise!
The surprise function is directly proportional to the log of the Probability.
Lower the probability is higher is the surprise.
There are some prior probability of things being happened. Or, chances of occurring certain things in certain ways. At least we have developed some kind of mental picture of forth coming events.
But there are always certain(non zero) probabilities of happening un-expected events. We are surprised when we encounter such events.
Links of interests: