The movie presents the experience of being possessed kind of like a drug. Mia describes the experience of her first possession in a way that sounds like people describing pleasurable drug experiences. Even people who have a bad time during their first possession, like Daniel, want to do it again and again. It's an addictive experience. Also, certain drugs can dilate people's pupils, and the eyes of people who are possessed are nearly completely black. The black eyes might be a visual allusion to drug use.