Every human soul is given a specific role in the world.
It could be said that each of us is allotted a “plot of land” in the world to “cultivate and keep” (see Genesis 2:15).
Every soul is required to fulfill the unique, specific role apportioned to it; only then will it be entitled to rest.
For all sorts of reasons – obstacles, delays, personal choice, failure to perceive our role – many of us do not manage to completely fulfill the soul’s mission during one lifetime.
A soul that has not fulfilled its role completely, and that has even harmed itself and others in the process, is sent back to the world in order to complete the task allotted to it.
If it doesn’t manage to finish its task the second time, it will be sent back again for as many times as it takes.
This return is referred to as a reincarnation, or gilgul.
Although usually a soul is returned to the world for the purpose of completing its unfinished task, occasionally it may be reincarnated for another reason – because either the world in general, or a specific individual, is in need of that particular soul’s contribution.
–Rabbi Adin Steinsaltz