Baal HaSulam, Shamati #53 “The Matter of Limitation”: And this is the meaning of, “any sorrow will be surplus,” meaning that sadness comes to a person because he wants luxuries. This is what it means that when Israel came to receive the Torah, Moses brought forth the people to the bottom of the mountain, as it is written, “and they stood at the nether part of the mount.”
(A mount [Hebrew: Har] means thoughts [Hebrew: Hirhurim]). Moses led them to the end of the thought and the understanding and the reason, the lowest degree there is. Only then, when they agreed to such a state, to walk in it without any wavering and motion, but to remain in that state as if they had the greatest Gadlut, and to be happy for it, this is the meaning of, “Serve the Lord with gladness.” This is so because during the Gadlut it is irrelevant to say that He gives them work to be in gladness, since during the Gadlut gladness comes by itself. Instead, the work of gladness is given to them for the time of Katnut, so that they will have joy although they feel Katnut. And this is a great work.
This is called the main part of the degree, which is a discernment of Katnut. This discernment must be permanent, and the Gadlut is only an addition. Also, one should yearn for the main part, not for the additions.
From this point he starts entering the spiritual world. An indication of this state is joy. First of all, one needs to reach the first restriction (Tzimtzum Aleph), the screen and the Reflected Light, the intention to bestow, and advance as far as possible forward. And then, everything that a person needs is not considered consumption. That is why the sign of this state is a constant joy because he does not need to worry about anything; he is bathed in the Light of the Creator, in the sea of bestowal.
From the Preparation to the Daily Kabbalah Lesson 2/5/13