The tendency to anticipate desired/undesired results to occur either one way or the other, without considering the risk of it turning out differently (either in the desired or undesired direction). This tendency includes the setup for an upset (and blame towards oneself, others, or the universe) if an anticipated desired results don't occur. If we anticipate an undesired result, then we can not only justify our unwillingness to try for the result but we can also avoid the risk of disappointment if we went for it. Moreover, if the anticipated undesired result did occur, it confirms the I-knew-it-wouldn't-be-good attitude, reinforcing a false belief that it could not have turned out desirably.
Expectations allow us to blind ourselves to the risks of it not turning out the way we count on as well as the possibility of it turning out the way we would like. They allow us to feeling safer, at the expense of putting us at more risk. They also allow us to indulge in "counting our chickens before they hatch." Ultimately, without expectations, we would never be upset and we would never feel betrayed or let down either by others or by ourselves.
Example: "I expected my girlfriend to be true to me. Then I caught her kissing my best friend. I was so betrayed."
By indulging in the expectation that he could count on his girlfriend to be loyal, he set himself up for betrayal. The Expectation bias could also be called the Hope bias, when related to positive expectations. See undoing expectations.