Social engineering is a tactic used by crackers, virus/worm creator, the viruses/worms themselves and others who want to gain access to a system with the help of the user. In the realm of self-replicating programs, social engineering is mostly applied to those worms and macros that spread through email.
Most commonly, social engineering exploits people's emotions and their desire to help others. Many worms and macros come disgused as love letters, threats from bill collectors, celebrity photographs, notices from financial institutions or auction sites, and even virus/worm alerts.
Since the first email-spreading programs, their creators know that in order to make the unsuspecting user run them, they would have to think of some sort of gimmick in order to entice the user to open it.