Still, [this second stage] is a great deal more advanced than the first one. Not only does a person acquire an understanding of the true greatness of the desire to receive; not only is all of materiality available to [him in order to] perform the service [of God] that he requires; but this is also the stage at which he can acquire [the quality of doing divine service] for its own sake. [As the Talmud explains, Pesaḥim 50b,] “A person should always engage in Torah and mitzvot, even if [he does so] not for their own sake, as through [the performance of mitzvot] not for their own sake, one comes to perform them for their own sake.”
That is why this stage, after thirteen years of age, qualifies as holy. This is the secret of the “sacred maidservant who serves her master,” which is the secret of the sacred Shekhina [Divine Presence; also a term for the lowest Sefira]. The maidservant brings the person to the level of “for its own sake” and therefore makes the person worthy of the revelation of the Shekhina.