When we come into this world, we are given the exact amount of time that is necessary to complete our spiritual work. Each of us has specific tasks to perform, and our tasks may be different than those of others; Kabbalah tells us that our transformation can be completed in one lifetime. We have enough time, but we also don't have any time to waste.
Unfortunately, most of us don't use our time very well. Everyday life distracts us from our larger purpose. As the years pass, we may achieve part of our transformation, or perhaps none at all. But the fact is, most of us depart this life without completing the task we came here to do. We may even have moved in the opposite direction, away from our purpose.
Kabbalah teaches that we will return to this world in many incarnations until we achieve complete transformation.
Work left uncompleted in this life is undertaken again in a future life until the task of transformation is done. Reincarnation is a fundamental tenet of Kabbalah. The world and our place in it cannot be understood without this key principle.
As always, what is true for a single human soul is also true for all humanity. As long as any one of us falls short of transformation, we will continue to participate in the cycle of birth, death, and rebirth - as will humanity as a whole - until the critical mass of truly enlightened people comes into being to eradicate pain and death forever.
The possibility of reincarnation is not a license to ignore our spiritual responsibilities in this life. On the contrary, reincarnation should be an incentive to complete our spiritual tasks as quickly as possible, to attend to the work of transformation, and to live more sharing, compassionate lives. Only in this way can we get off the wheel of arrival and departure in the world, and get rid of the pain that inevitably comes with it.