1 The brain filters repetitive signals
The brain reduces the load on its cells by filtering repetitive signals. When an unexpected thing happens it grabs our attention as the brain pays extra attention to it.
2 Unexpected things make us laugh
According to psychology, one of the things that make some jokes funny is that they have unexpected twists.
3 What is predictable can be boring
A predictable thing can be considered boring. If a person knows everything about something or if they can predict everything about it then they can easily get bored.
4 Unexpected rewards are addicting
According to addiction psychology, unexpected rewards are one of the important components of addiction.
5 The low expectations increase happiness
As a person gets suddenly surprised by a reward they didn't anticipate, they become happy. Expectations can sometimes lower happiness levels or even cause disappointment.
6 People get attracted to mysterious ones
According to attraction psychology, people find mysterious ones much more attractive than predictable ones. (See How to look mysterious?)
7 The pleasure center in the brain gets more active
In one study it was found that the pleasure center in the brain becomes more active when a person gets subjected to an unexpected reward.
8 They let us anticipate more rewards
When a person gets an unexpected reward they might start anticipating more rewards thus releasing more dopamine. Dopamine is a chemical released when people anticipate rewards and it's responsible for the excitement and pleasure people get when they expect something good.
9 Our moods depend on the rewards we expect
Our moods greatly depend on the rewards we expect. When something pleasant happens unexpectedly our mood gets a strong boost because the pleasure wasn't anticipated. (See Why novel and bizarre things catch your attention?)