Reincarnation is an idea that may seem strange, exotic, or even a little nonsensical to the western mind, but in fact it plays a vital role in understanding our purpose in the universe and why we spend time on this planet at all. In fact, it is almost impossible to understand how spirituality works in any practical sense without touching upon the subject, for it is the mechanism that not only drives spiritual growth but makes it possible in the first place.
To understand it best, it is a good idea to contrast reincarnationist beliefs with the traditional western belief that the soul is a "one-shot" affair that comes into existence at the moment of conception (or perhaps shortly thereafter) and then exists throughout the balance of eternity, either in a state of spiritual bliss—which we call Heaven—or spiritual torment or separation, which we generally refer to as Hell. With this concept, the main purpose of the physical world seems to be to "birth" new souls into existence, where they are then free to reject or embrace God and so determine their eternal fate, making the physical realm, in effect, a type of "nursery" where souls are "hatched" and briefly incubated before winging their way to their final spiritual state.
The problem with this concept is obvious: if we have just this single lifetime within which to turn to the Creator and so realize Heaven (be saved) what of those who are born into circumstances from which conversion to the "proper" faith is all but impossible? For example, a man born in sixteenth century Iran is almost guaranteed to become a Muslim, disqualifying him from the salvation guaranteed only by conventional Christianity. (This problem works the same from a Muslim perspective as well, for if paradise is the reward for being a good Muslim, salvation would be difficult for the infidel born in a Christian country to realize.) Thus, the idea that the human soul has only the single lifetime within which to find God and the "correct religion"—especially when one takes into account the vast odds against it being born into the right time and place or having the proper disposition—is not only impractical but illogical.
Additionally, beyond mere conversion, the idea that we are to use this short time in the physical to not only "find God" but mature towards spiritual adulthood is difficult to accept. Even if one was to have the good fortune of being born into the ideal set of circumstances, even a century would still be too short a time to realize more than a handful of potentially valuable spiritual lessons. Regardless of how compassionate or spiritually mature one may appear, their experiences are but one perspective of many.
For example, suppose a man is born into a white, Protestant family in America in the 1960s and, while a decent person from all outward impressions who has made a determined effort throughout the course of his life to be tolerant of others and understanding of other races, cultures and beliefs, from the perspective of eternity, what has he really learned? Has he ever experienced what it is to overcome the unique problems of being a single mother or confronted the obstacle of racism? Has he ever seen the world from the perspective of a jail cell or lived for one moment in abject poverty? Does he know what it is to live the life of a street prostitute—alone, ostracized, brutalized, and rejected by society as a worthless human being—or, for that matter, can he honestly say he understands the fear that drives men and women to commit acts of great brutality, seeing that his entire perspective is shaped from within the context of his singular cultural perspective, gender, race, or religion? In essence, can he truly understand the day to day problems, pains, and challenges of life that exists outside of his own unique perspective—that "little world" he and, the truth be told, we all—create for ourselves?
Of course not. No one could. Regardless of how compassionate or spiritually mature a person may appear, their experiences are but one perspective of many; a single drop of human experience in a sea of individual human dramas. Even in the most perfect circumstances—a situation in which few people would likely find themselves—that are ideal towards achieving the maximum level of spiritual growth, a long and full lifetime is not enough time to do more than scratch the surface of spirituality. As a result, most of us are born into a world in which achieving spiritual enlightenment is hardly deemed a serious endeavor, even if we had the time or disposition to pursue it. Instead, we are born into circumstances beyond our control, confined within a specific time and cultural context, and ushered into a family that is as likely to be as dysfunctional as enlightened. Yet, within this brief experience we call life we are expected to—at least according to western tradition—not only find the "correct" path to God but also grow into spiritually mature men and women in the process. One might as well expect a toddler to assume the reigns of government and exercise only the wisest and most insightful leadership.
This is the very problem reincarnation addresses. It understands that a single lifetime is far too short a time to achieve much of value, even under the best of circumstances. As such, though it shares with western orthodoxy the idea that the soul is immortal and eternal, it also perceives it as being preexistent and in a state of flux. It understands the soul to be that aspect of ourselves that periodically takes on the mantle of flesh, lives out a brief physical lifetime, and then returns to the spirit realm to reflect upon the lessons of that life before assuming the flesh again in a new incarnation. It is not interested in simply existing in the physical universe for one brief period of linear time and then residing for the balance of eternity in the spiritual realm, but in making the trip over and over again in an effort to acquire new experiences and to mature towards greater spiritual perfection. It sees earth—or the physical realm in general—as a "testing ground" where spiritual concepts are tried out and obstacles overcome in an effort to grow and mature spiritually, something the traditional "one shot" idea of soul creation is incapable of doing. As such, the reincarnationist fully understands the purpose of the physical realm to be "where the action is." In that regard then, reincarnation gives the physical universe its meaning and, more importantly, its purpose. It is the place where we are spiritually perfected, making it far more than a mere "way station" on the way to our eternal rest; it is, instead, more like a university of higher learning we must visit frequently if we ever hope to acquire anything approaching a well-rounded spiritual education.
So what is the underlying purpose of doing this? Moving in and out of the physical realm through a series of rebirths is what makes it possible for God to effectively experience "Himself" in a practical way rather than in a purely conceptual way—something He is incapable of doing from the realm of pure spirit—and, in living through a vast array of personal experiences—both positive and negative in nature—the process slowly reunites—or, more correctly, reintegrates—the soul that birthed us with God. The reason it does this is two-fold; first, it does this because it is naturally drawn to the Creator (or, more precisely, Source) as a result of being a part of God and, second, as it learns—or, actually "relearns" what it always knew inherently—that love is the only path to eternal bliss and the Creator is the source of that love, it looks to reunite itself with the source of all love, which it does, paradoxically, by experiencing what it is to be without love. In effect, the soul can only experience love by putting itself in situations where it does not know love, and that experience ultimately propels it towards the source of all love—God. In so doing, it demonstrates that life never ends any more than God could have an ending. In fact, reincarnation is what gives life its immortality. It is life itself—never ending—and the truest testament to the immortality inherent within each human being.