People smirk when they feel superior, self-satisfied or conceited. They may also smirk and take pleasure in knowing that someone they're envious or jealous of is suffering.
People who smirk are often those with excessive self-admiration and self-centeredness. Because of these narcissistic traits, they smirk, indicating that they think they're better than the ones at whom they're smirking. Such behavior is similar to laughing with contempt and other derisive looks or remarks. On the other hand, politicians and other public figures may hide their rage and anger through smirking to protect their reputation. They have public scrutiny to watch out for and must hide their aggressiveness through seeming smiles.