The Zohar, Chapter “VaYechi (And Jacob Lived),” Item 157: How much longer was I given to live in this world? I have no permission to inform you this, and a man is not 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.
The death of a righteous person resembles the purification (Hizdakchut) of a Partzuf. When a person leaves his degree and transitions from one world into the other, and especially when he leaves the lowest of all degrees – this world – it is a big joy.
However, we think otherwise, and believe that we live in a wonderful world. We simply haven’t seen anything better besides this world. We have to reveal the spiritual world, as it is said, “See your world in this life.” Then we will realize where in fact we are now.
If a person completes his mission, he doesn’t have to return to this world as it happened to Rabbi Shimon who attained the final correction. From now on he stays only in the higher dimension and doesn’t reincarnate into the lowest material form. Whoever hasn’t achieved the final corrected state in a given reincarnation has to be reborn and live in this world again.
It goes without saying that Rabbi Shimon is blissful not because he finally got rid of the work and can now enjoy rest but because he brought the Torah (the Light that Reforms) to the whole world. This of course overwhelms him with joy.