Motivation affects perception. If I gave you a perceptual problem to solve you might keep at it for a short time. But, if at the end of that time you still had not solved the problem you might give up. But, if I offered you 20 bonus points toward your final grade, you would probably keep at it for a little longer.
All of us bring a wealth of expectations to any perceptual situation. We expect that fly balls in a baseball game will eventually come down. We expect that a sentence will have a subject, a verb, and an object. We know what we are trying to write, so proof reading our own material is difficult. We may perceive things that fulfill those expectations.