Tanya Chapter 37 part 5

The soul is perfect, requiring no rectification. What the soul had above in terms of love and fear of Hashem was way beyond anything it could achieve in this world. Even if the soul will reach the highest levels humanly possible –– that of a complete tzadik who loves Hashem with pleasurable love –– it will not experience anything remotely similar to what it experienced while above. The whole purpose of its descent is only to elevate the vital soul, the body, the world. We cannot understand this –– it is a mystery paralleling the mystery of the exile of the Shechina. It too is exiled, coming down to elevate holy sparks.