תּוֹרָה קכו

As soon as Shabbos enters — from the great delight of the additional soul [nefesh yeserah] that comes on Shabbos — one immediately begins to long and grieve over the loss of the soul at the close of the holy Shabbos.

And this is the aspect of ""it rested and was refreshed"" [shavat vayinafash] that is said at the onset of Shabbos — and our Sages of blessed memory expounded: ""once it rested — woe [vay] — the soul is lost"" [Beitzah 15b].

And this is similar to what is brought in the holy Zohar — that when the "Chavrayya" [disciples] greatly delighted from the holiness of the revelation of Rabbi Shimon bar Yochai — they began to long greatly — and said: ""woe to the generation when you depart."" And this is exactly like the above.