The tzaddik who descended to this world had to part from all the souls who were together with him in Gan Eden.
As long as he remains in this world, these souls yearn for him.
When he departs this world and returns to his original place, these souls hold a celebration and rejoice over the reunion.
The implication is that this supernal joy over the reunion is so great that it overcomes the sorrow felt by people in this world over the tzaddik’s death.
They too, overcome their sorrow and share in the soul’s joy at having reached a happier place.
Thus, an ancient text records a eulogy from Talmudic times:
“Weep for the mourners, but not for the departed; for he has gone to rest, and we – to lament.”
All the more so in the case of R. Shimon bar Yohai, who at his death requested that people not mourn for him but rather rejoice over the ascent of his soul.
–Rabbi Adin Steinsaltz