G-d told Moses to tell the Jewish people: "You must celebrate a pilgrim festival for Me three times a year."
The pilgrim festivals mark the three significant milestones in the agricultural cycle: Passover occurs in the spring, when the produce begins to ripen; Shavuot occurs in the early summer, when the wheat is harvested; and Sukkot occurs in the fall, when all of the produce is gathered in from the fields.
Allegorically, the Jewish people are G-d's "produce." Just as one sows grain in the hope of reaping a much greater return, G-d "Plants" souls in the physical in order for them to accomplish much more than they can in their native, heavenly abode.
When we plant a seed, it does not begin to grow immediately. Growth can begin only once the outer, protective coating of the seed disintegrates. Once the original seed per se no longer exists, the new growth is not restricted by the limitations of the original form of the seed.
The same is true of human growth: ego is its greatest hindrance. Only when we overcome and negate the ego can the soul reach its full potential.