Cialdini's book, Influence, the Psychology of Persuasion, outlines the various ways in which we are persuaded by others because we have a biological need to fit in, be part of the group and thereby overcome our fears and expedite our gain or comfort.d
When we give something away for free, (say a one free lunch, information, help) most people feel an explicable desire to reciprocate.
Consistency builds trust - it does so through familiarity. The more we are exposed to something, the more we associate with it (warning - positively or negatively).
When we want to be like everyone else.
When the "attraction, warmth security and the obligation of friendship are brought to bear on the sales setting."
Robert Cialdini, Influence
Deference to authority causes us to act, not necessarily to understand why.
Get it now, they are going fast!