The middle voice is used to express an action done by the subject done in his or her own interest.

Do not think of the middle as the reflexive (as in the sentence "He cut himself"). Wenham (p. 93) says that the middle is more like the active than like the passive — a "sort-of-Active." Classical Greek usually would use the active voice with a reflexive pronoun. In koine Greek, however, the middle sometimes functions as the reflexive.

"He cut himself." (active voice).
"He cut wood [for himself]." (middle voice)