"Escape from emptiness, depression, and feelings of unreality.
Providing relief: when intense feelings build, self-injurers are overwhelmed and unable to cope. By causing pain, they reduce the level of emotional and physiological arousal to a bearable one.
Relieving anger: many self-injurers have enormous amounts of rage within. Afraid to express it outwardly, they injure themselves as a way of venting these feelings.
Escaping numbness: many of those who self-injure say they do it in order to feel something, to know that they're still alive.
Grounding in reality, as a way of dealing with feelings of depersonalization and dissociation
Maintaining a sense of security or feeling of uniqueness
Obtaining a feeling of euphoria
Expressing emotional pain they feel they cannot bear
Obtaining or maintaining influence over the behavior of others
Communicating to others the extent of their inner turmoil
Communicating a need for support
Expressing or repressing sexuality
Expressing or coping with feelings of alienation
Validating their emotional pain -- the wounds can serve as evidence that those feelings are real
Continuing abusive patterns: self-injurers tend to have been abused as children.
Punishing oneself for being "bad"
Obtaining biochemical relief: there is some thought that adults who were repeatedly traumatized as children have a hard time returning to a "normal" baseline level of arousal and are, in some sense, addicted to crisis behavior. Self-harm can perpetuate this kind of crisis state
Diverting attention (inner or outer) from issues that are too painful to examine
Exerting a sense of control over one's body
Preventing something worse from happening
These reasons can be broadly grouped into three categories:
Affect regulation -- Trying to bring the body back to equilibrium in the face of turbulent or unsettling feelings. This includes reconnection with the body after a dissociative episode, calming of the body in times of high emotional and physiological arousal, validating the inner pain with an outer expression, and avoiding suicide because of unbearable feelings. In many ways, as Sutton says, self-harm is a "gift of survival." It can be the most integrative and self-preserving choice from a very limited field of options.
Communication -- Some people use self-harm as a way to express things they cannot speak. When the communication is directed at others, the SIB is often seen as manipulative. However, manipulation is usually an indirect attempt to get a need met; if a person learns that direct requests will be listened to and addressed the need for indirect attempts to influence behavior decreases. Thus, understanding what an act of self-harm is trying to communicate can be crucial to dealing with it in an effective and constructive way.
Control/punishment -- This category includes trauma reenactment, bargaining and magical thinking (if I hurt myself, then the bad thing I am fearing will be prevented), protecting other people, and self-control. Self-control overlaps somewhat with affect regulation; in fact, most of the reasons for self-harm listed above have an element of affect control in them.
In an interesting theory that combines all three categories, Miller (1994) posits an explanation for why such a large majority of peep who self-harm are female. Women are not socialized to express violence externally and when confronted with the vast rage many self-injurers feel, women tend to vent on themselves. She quotes the feminist poet Adrienne Rich:
Alexithymia is a fairly recent psychological construct describing the state of not being able to describe the emotions one is feeling. Alexithymia was positively linked to self-injurious behavior in a 1996 study (Zlotnick, et el.) and is congruent with how people who self-injure often describe the emotional state before an injury; they frequently cannot pinpoint any particular feeling that was present. This is especially important in understanding the communicative function of self-injury: "Rather than use words to express feelings, an alexithymic's communication is an act aimed at making others feel [those same feelings]" (Zlotnick et al., 1996).
Self-capacities and Invalidation
A constructivist theory of self-injurious behavior (Deiter, Nicholls, & Pearlman, 2000) holds that people who self-injure usually have not developed three important self-capacities: the ability to tolerate strong affect, the ability to maintain a sense of self-worth, and the ability to maintain a sense of connection to others. The first of these speaks directly to the affect-regulation role of self-harm; the others are perhaps related to its communicative functions.
Pearlman et al. (2000) note that "when children experience shaming and punitive rhetoric or physical blows rather than responsive words" they cannot internalize others are loving and cannot develop the capacity to maintain a sense of connection to others. They further state, "The ability to experience, tolerate, and integrate strong affect cannot develop fully when strong feelings are met with punishment or derision." Having a sense that some feelings are unacceptable and not allowed also impairs this ability. And the ability to maintain a sense of oneself as a person of worth cannot be developed when a child never feels she is good enough, when her "existence and accomplishments are met with silence or abusive words or actions."