We all know it's easier to remember things that surprise us, but what is it about surprise that makes things memorable? A recent paper suggests that surprise does not simply equate with low probability, although low probability is generally associated with surprise. The authors argue that the level of surprise is chiefly determined by the degree of contrast between the (surprising) event and the expected event. This brings the memorability of surprise back to the tried and true principle of distinctiveness - we remember more easily those things that are distinct; the more distinct, the more easily remembered.
But there's something else that might be going on, too. An imaging study has showed, to everyone's surprise!, that neural reward pathways - "high-speed Internet connections to the pleasure centers of the brain" - respond more to unpredictable events than pleasurable ones. In other words, we are designed to find surprise rewarding.
Another excellent reason for surprise to be memorable.