Control

From "Why Gamers Can't Stop Playing First-Person Shooters," by Maria Konnikova, New Yorker Online. Emphasis mine.

Control, compounded by a first-person perspective, may be the key to the first-person shooter’s enduring appeal. A fundamental component of our happiness is a sense of control over our lives. It is, in fact, “a biological imperative for survival,” according to a recent review of animal, clinical, and neuroimaging evidence. The more in control we think we are, the better we feel; the more that control is taken away, the emotionally worse off we become. In extreme cases, a loss of control can lead to a condition known as learned helplessness, in which a person becomes helpless to influence his own environment. And our sense of agency, it turns out, is often related quite closely to our motor actions: Do our movements cause a desired change in the environment? If they do, we feel quite satisfied with ourselves and with our personal effectiveness. First-person shooters put our ability to control the environment, and our perception of our effectiveness, at the forefront of play.