The Arizal says — that a neshama needs to keep coming back into this world (gilgul) in order to fulfill all of the 613 mitzvos that it is possible for the neshama to keep. The Alter Rebbe explains to us why the neshama needs to have all 613 mitzvos, because they are the “clothes” for all 613 parts of the neshama to appreciate the light of Hashem in Gan Eden.