See BBG 25.15 or GGBB p. 414ff. Young writes:
“The middle voice is difficult to express in English, since English does not have a corresponding idiom. The student should avoid the idea that it is always reflexive (I loose myself). The basic notion is that the subject intimately participates in the results of the action. It is the voice of personal involvement. Even though deponent verbs are translated with an active sense, they often convey the idea of interest or involvement.” Young, Intermediate New Testament Greek, 134.
(a) ORIGIN OF THE MIDDLE. See chapter VIII, VI, (c), for the uncertainty as to the priority of active and middle. That question is an open one and must be left open. Both active and middle appear in Sanskrit and in Homer. The prehistoric situation is purely speculative. Logically the active would seem to come first, though the difference in form may be due to variation in sound (ablaut). Probably at first there was neither active nor middle, the distinction being a development. In the Sanskrit we meet a full system of both active and middle forms for all the tenses (not all the modes), the participle, however, having only a partial system and the infinitive no voice at all. But each verb has its own development and that was by no means uniform. Some had a very limited use as to voice, tense and mode. In Homer indeed the middle is rather more common than in later Greek. It is only in the Sanskrit, Zend (Old Persian), Greek and Gothic that the middle is kept as a distinct voice. In the Gothic only remnants of the middle are found, while in Latin the middle as a separate voice disappears. It is very difficult to run a parallel between the Latin and Greek voices. But there is a considerable remnant of Latin middles like miror, sequor, utor (cf. Draeger, Hist. Syntax, pp. 145 ff.). The final disappearance of the Greek future and aorist middle before the passive is well sketched by Jannaris. But at first we are not to think of the passive at all, that interloper that finally drove the middle out of use.
(b) MEANING OF THE MIDDLE. It is urged that the term “middle” is good because the voice in meaning stands between the active and the passive. But, unfortunately for that idea, the middle is older than the passive. It is true that the passive arose out of the middle and that the middle marks a step towards the passive. The passive idea existed before there was a separate passive form, a thing never true of all tenses and all verbs. The Hebrew Hithpael conjugation is somewhat parallel, but not wholly so. The only difference between the active and middle voices is that the middle calls especial attention to the subject. In the active voice the subject is merely acting; in the middle the subject is acting in relation to himself somehow. What this precise relation is the middle voice does not say. That must come out of the context or from the significance of the verb itself. Gildersleeve is clearly right in holding that the interpretation of the difference between active and middle is in many cases more lexical than grammatical. “The middle adds a subjective element.” Sometimes the variation from the active is too minute for translation into English. This “word for one’s self” is often very difficult of translation, and we must not fall into the error of explaining the force of the middle by the English translation. source