Do we only live once? It all depends on one's perspective. If you think we're nothing but chemicals with no soul, then yes, it is the end. If you believe in only one life and then it's heaven or hell, then no. It's especially not the end if you happen to believe in reincarnation. Reincarnation suggests this is not our first life on Earth and it's most likely not our last. The Hindu perspective suggests we have been around for millions of lifetimes and that there may be millions ahead. In that sense, we are all "old souls."
What is it that reincarnates and how exactly does it happen?
The Bhagavad Gita, the ancient text of India suggests that the soul leaves when the
body dies. The type of body and psychological disposition -- likes, dislikes, phobias, etc., one receives upon rebirth is dependent on the actions (karma) within that one life combined with the actions from previous lives. By some inconceivable system, all of one's positive and negative deeds are recorded. This means, no one gets away with anything. Moreover, all positive and negative actions create within the individual a psychological disposition to continue performing that action. For example, if one starts stealing or gets in the habit of lying and doesn't try to change these tendencies, then in the next life, one will pick up where they left off. The same goes for positive actions.
The mind propels us into action based on its conditioning which is based on past actions. So, when the body reaches its demise, the mind and soul get transferred into, hopefully, another human womb. Even while in the womb, the mind remains active and is recalling events from the previous life. In a NYTimes article "Do Babies Dream," Charles P. Pollack, director of Center for Sleep Medicine at New York-Presbyterian/Weill explains, "Most dreaming occurs during a type of sleep called REM sleep...which '...occurs at all life stages, including infancy, and even before infancy, in fetal life.'"
This article doesn't establish the fact that the dreams of the fetus are from previous lives, but it does indicate that the little guy is dreaming. If he or she hasn't lived before, what in the world could it be dreaming of?
The mind functions kind of like a hard drive with millions of files on it. If a hard drive is transferred into a different computer, the new computer will have all the same information as the previous one. So, when the mind and soul are relocated into a new body, the exterior is different, but all the programming stored in the mind are exactly the same. This programming will make the person behave in very much the same way as they did in the previous life because the conditioning and impressions are the same. The car may be different, but the driver is the same. Hence, we are not very different from who we were in our previous life.
This might help to explain the phenomena of children with past life memories. The memories are very specific and can often be verified. Dr. Ian Stevenson, the founder and director of the University of Virginia's Division of Perceptual Studies researched and wrote about this phenomena extensively.
Most adherents of the Hindu and Buddhist traditions will have little difficulty accepting that the soul moves through different bodies until it ultimately purifies itself of anger, greed, envy and pride. This might be because life is seen as a cyclical phenomena in the East as opposed to a linear one in the West. All the intricacies of karma and reincarnation will always remain mysterious to the human mind, but knowing that I will get a second or third chance, gives me hope.