Surplus of supernal holiness is holy – Hairs of the Nazir: An example of holy surplus of supernal holiness can be found in the 13 attributes of mercy from which there are spiritual hairs that grow. These spiritual hairs are the surplus of this great level of holiness and is itself holy, not giving any nurture to the unclean side. This is the level of hairs represented in the hair of the Nazir of which it states that “Holy will be his overgrowth of hair of his head.”
Surplus of lower level holiness nurtures the unclean side – Hairs of the Levites: An example of an unholy surplus of holiness, which comes from a lower level of holiness, can be found by the Levites. The Levites below are rooted in the level of harsh severities above. [Now, while the harsh severities in which they are rooted is holy, and comes from Gevura of Atzilus, nonetheless, it’s surplus is unhealthy and can lead to unwanted nurture to the side of evil.] Accordingly, the Levites were instructed to shave their hair [during their inauguration process in which all the hair of their body had to be shaved clean] in order to get rid of the surplus of harsh severities of holiness, which were represented in their hair.
Hairs of a married woman:[1] It is also due to the above reason that a married woman is considered an Erva and she is hence required to cover her hair, in order to prevent the hair from giving nurture to the unclean forces. [Women come from the level of severities, similar to Levites, and hence their hair represents a surplus of severities similar to that of the Levites. Hence, just as the Levites had to shave their hair to prevent the unhealthy nurture of evil that can derive from the surplus, so too, a married woman must cover her hair to prevent this. Now, as for why a married woman is not required to shave her hair similar to the Levites, this is because a certain limited level of severities are necessary for this world to function, and hence her hair is not to be shaved but merely covered.[2] As for why specifically a married woman, as opposed to a girl who was never yet married, is required to cover her hair, seemingly, this is because the aspect of severities only becomes expressed in a woman after the union of marriage.[3]]
[1] Likkutei Torah Ki Seitzei p. 36b; See also Likkutei Torah Beshalach 1a; 24a; Emor 32a; Nitzavim 52a; Derech Mitzvosecha Mitzvas Tiglachas Metzora 104b; Likkutei Sichos 18:447; Igros Kodesh 15:415
[2] See Sefer Shaar Hamitzvos of Arizal on Parshas Kedoshim “However a woman comes from the level of Za… And all of the grasping of the severities and their root is in her, and if you remove the severities completely from the head of the female then they will be completely nullified, and therefore a woman is not to shave her head and on the contrary she’s to grow out her hair as is well known, to leave a root for the severities”; Ben Yehoyada Miseches Brachos p. 61 and Miseches Yuma p. 47; Shemen Sasson 9
[3] See Likkutei Torah Parshas Tazria in beginning that specifically in marriage a woman’s Gevurah is expressed, although as Chesed Shebegevura