There is no permission to inform a person how much longer is one given to live in this world; one is not to be informed of this. But Rabbi Shimon was in great joy on the day of his demise, and there was great joy in all the worlds because of the many secrets he had revealed then (Rashbi, Zohar for All, Vol. 3, VaYechi, item 157, “When His Time Comes to Depart from the World”).
The departure of a righteous person is like the purification of a Partzuf. When a person leaves their degree and passes from world to world, and especially when leaving the lowest degree, this world, it is a great joy.
Although it does not appear so to us and we think we are living in a wonderful world, we simply have not yet seen anything else besides it. But one must reveal the spiritual world, as it is said: “You shall see your world in your lifetime,” and then we will understand where we truly are.
If a person completes their mission, they no longer need to return to this world, as was the case with Rabbi Shimon bar Yochai, who attained final correction. This means that from then on, he exists only in the upper dimension and no longer reincarnates into this lower, corporeal form.
But one who has not reached their final corrected state in this incarnation must be born again and live in this world.
It is clear that Rabbi Shimon bar Yochai rejoices not because he has finished his work and can rest, but because he brought the Torah, that is, the light that reforms, to the whole world. And this, of course, fills him with joy.
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From a lecture for the holiday of Lag Ba’Omer, 5/1/10



