How do we know how the book is going to be before we read it?

How do we know the movie we are paying $25.00 for is going to be worth it?

How do we know that the food at that new restaurant we are trying out will be good?‍

The answer is that we don't. So built into marketing is the idea of expectations - that there is a promise being made before we sell something.
So before we even sell something we have the opportunity of asking: what are the expectations at? If they are too high people will be disappointed. If they are too low people won't consider taking action.
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The holy grail of setting expectations is the following: setting expectations in such a way so that it's exciting enough for me to take action but also set in a way that I will be over delighted when I finally have the experience - because they exceeded my expectations.

As the brand develops the baseline of expectations shift and people forget this in marketing. If you do the same thing over and over again without shifting the baseline - you start to move backwards in your marketing.
Remember what are the expectations that people have of this product or service? How can I set them? And how are these expectations changing overtime?