C. F. Keil and Franz Delitzsch on Numbers 6:5

Num 6:5 Secondly, during the whole term of his vow of consecration, no razor was to come upon his head. Till the days were fulfilled which he had consecrated to the Lord, he was to be holy, “to make great the free growth (see Lev 10:6) of the hair of his head.” The free growth of the hair is called, in Num 6:7, “the diadem of his God upon his head,” like the golden diadem upon the turban of the high priest (Exo 29:6), and the anointing oil upon the high priest’s head (Lev 21:12). By this he sanctified his head (Num 6:11) to the Lord, so that the consecration of the Nazarite culminated in his uncut hair, and expressed in the most perfect way the meaning of his vow (Oehler). Letting the hair grow, therefore, was not a sign of separation, because it was the Israelitish custom to go about with the hair cut; nor a practical profession of a renunciation of the world, and separation from human society (Hengstenberg, pp. 190-1); nor a sign of abstinence from every appearance of self-gratification (Baur on Amo 2:11); nor even a kind of humiliation and self-denial (Lightfoot, Carpzov. appar. p. 154); still less a “sign of dependence upon some other present power” (M. Baumgarten), or “the symbol of a state of perfect liberty” (Vitringa, obss. ss. 1, c. 6, §9; cf. Num 6:22, Num 6:8). The free growth of the hair, unhindered by the hand of man, was rather “the symbol of strength and abundant vitality” (cf. 2Sa 14:25-26). It was not regarded by the Hebrews as a sign of sanctity, as Bähr supposes, but simply as an ornament, in which the whole strength and fulness of vitality were exhibited, and which the Nazarite wore in honour of the Lord, as a sign that he “belonged to the Lord, and dedicated himself to His service,” with all his vital powers.In support of this explanation, Oehler calls to mind those heathen hair-offerings of the Athenian youths, for example (Plut. Thes. c. 5), which were founded upon the idea, that the hair in general was a symbol of vital power, and the hair of the beard a sign of virility; and also more especially the example of Samson, whose hair was not only the symbol, but the vehicle, of the power which fitted him to be the deliverer of his people.