The personal need for justice is something that is inherent in most men and women, and when one suffers an injustice, there is a natural urge to make the situation right. This is often a reason for revenge. Reversing the injustice in some manner can be highly fulfilling, thus the term "sweet revenge."
However, vengeance is often looked upon as a harmful and unethical act, since the goal is often to inflict greater punishment to the person who committed the original wrong. Personal vengeance also goes against the true meaning of justice, when punishments are handed down in a legal fashion. Nevertheless, revenge can be sweet, especially in literature, as evidenced by Montresor in Edgar Allan Poe's "The Cask of Amontillado," who enacts the ultimate revenge--murder--upon the man who had "borne" him "a thousand injuries."