Since the target of Social Engineering is a person, the success of this sort of attack highly depends on the human factors. Specifically, several truths about people increase the chances for a successful Social Engineering attack:
Pages 309 - 312 of the Nayak & Rao textbook include several scenarios exepmplifying attacks that take advantage of each of the above traits.
Policies, awareness, and trainings are the only major means of ensuring that the people are not easily conned through social engineering attacks.