There is an old saying that captures the essence of the reciprocity norm: "eye for eye, tooth for tooth". When we receive something we have a tendency to "pay back the debt" and give something in return. It is both true for good things and bad things.
Advertising uses this norm quite often: they give you little gifts, a bite of a product to taste and hope that by accepting the gift, you will have an internal push to reciprocate.
Here is a short and funny video from the Office series to show you how it works: