The (Not So) Good Old Days
The Kabbalists explain further that the lifespans of the masses is in accordance with the "size" and spiritual potential of their souls.
This explains why the initial generations, such as Adam's and those listed toward the beginning of the Torah, lived much longer lives than we do.
Only as the spiritual capacity of the generations diminished did people's lifespans dwindle to 70 to 120 years. The reason for this is because earlier generations had "larger," more inclusive souls and therefore needed, and were given, more time to fix and rectify their portion of the sin. Only once they did not utilize their long lives for this purpose (since when one has all the time in the world he is much less motivated to act) were their souls further fragmented into "smaller" souls, each with less to fix.
This made it "easier" for each individual to rectify their portion of the sin and to fulfill his unique individual purpose, while simultaneously increasing his motivation to act since the lifespans of the masses were now shortened.
The Best of Both Worlds
Ultimately, in the end, all souls will return to that original higher level of unity and oneness yet will retain the unique individualistic expression of self that they managed to achieve while in their fragmented state; they will simultaneously be in touch with the state of "wholeness" and the state of "split-upness" in an experiential state of simple oneness.
Author's Note: Rabbi Eli will be traveling across North America in February 2012 to present his empowering and impactful Kabbalah seminars on Spirituality, Relationships, and Self-Help. For a complete catalog or to book a life-altering event contact firstname.lastname@example.org.