1 People feel safe around what they know
People prefer the things they know over the things they don't understand as this can help them feel safer (See why we like familiar things).
2 Uncertainty opens the door for bad possibilities
When a person becomes uncertain his mind might start to think of the possible bad events that could happen. Uncertainty thus could lead to bad moods.
3 Some people like to be in control
People who always want to be in control of everything usually hate uncertainty since it takes control away from them.
4 Investors only put money when they are sure
Investors usually move away from uncertainty by putting money in asset classes that they are certain about (See also the psychological factors affecting the stock market).
5 Uncertainly keeps the brain working
According to a theory called the Zeigarnik effect the brain keeps thinking about issues as long as there is incomplete information. Because uncertainty usually means that there is incomplete information it can result in unpleasant feelings.
6 Fear of the unknown
Many people fear the unknown and when they become uncertain about something their fears grow bigger.
7 Uncertainty fuels anxiety
When a person becomes uncertain about something that he cares about he starts to become anxious. some people hate uncertainty because it brings anxiety.
8 Uncertainty leads to rumors
When people become uncertain about something the possibility of spreading a rumor becomes much higher.
9 Pessimists might think about the worst possible scenario
When they become uncertain pessimists might start thinking about the worst possible scenario.