Expectation is not based on reality, and anything not based in reality is fantasy. When something goes unexpected we think, in some variation, "it 'should' not have happened". Reality is it did, but we're upset about it anyways.
It's not because of the result itself but because of disappointment. When you have expectations, naturally it leads to a fear of disappointment. That fear develops into numerous problems overtime in various aspects of our lives, and society.
Clarify: Disappointment is a form of mental (ig "emotional") pain, typically we avoid, cope, or act aggressively to painful things.
In the case of the disappointment, we tend associate the pain of disappointment to the person/thing that triggers our response. In reality we do it to ourselves, because our expections is based on assumptions and beliefs. To cut this short, people project their anger at themselves towards others when something doesn't goes the way they deem it should. I'm not sure how to prove it, but I think expectation is taught to us from our environment, not an internal mechanism like joy or grief.
No matter how old we get we'll always be a big baby, some think hiding the fact from other makes them "mature". If this all makes any sense.