Do you believe in reincarnation? If so, please share more information about this concept. Do the reincarnated Jews know that they lived earlier for instance (since it was a lesson for them)?
There are many Jewish sources dealing with what is popularly called "reincarnation." In Hebrew, it is called "gilgul ha'ne'shamot," literally the recycling or transmigration of souls.
This concept can be compared to a flame of one candle lighting another candle. While the essence of the second flame comes from the first one, the second flame is an independent entity.
Still, the new flame contains imperfections inherited from the initial flame, and it is these imperfections that are to be corrected.
Most of the written material is very esoteric, often written in Aramaic. Some of the prominent works dealing with this subject are the "Zohar" (1st century) and the Arizal's "Shaar HaGilgulim" (16th century). In the Bible itself, the idea is intimated in Deut. 25:5-10.
Many sources say that a soul has a maximum of three chances in this world. One example given is that the great Talmudic sage Hillel was a reincarnation of the Biblical figure Aaron.
The soul only comes into this world in the first place in order to make a spiritual repair. If that is not fulfilled by the end of one's lifetime, then the soul will be sent down once again. The return trip may only be needed for a short time or in a limited way. This in part explains why people are born with handicaps or may live a brief life.
It is not necessary that there be a conscious awareness in order for the correction to take place. Conscious awareness is only one level of understanding.
This idea is explored in an interesting book called "Psychic Phenomena," by Dorothy Bemar Bradley, M.D., and Robert A. Bradley M.D.:
"Mentally retarded children have been known to burst out with unexpected abilities under altered awareness, manifesting the contents of the undamaged and theoretically undamageable unconscious mind."
In other words, there are levels of understanding that transcend the conscious level, even in children.
Re: your second question. Why does this have to involve the body in the first place?
Truly, some "corrections" do not have to take place through the body, but rather take place in the soul world, in the afterlife.
However, sometimes the correction must occur in the physical world. For example, it may involve a certain challenge of choosing the "right thing" over choosing the "comfortable thing." Or other people may have to be involved. And the soul cannot interact with the physical world in any other way expect through a body.
The bottom line in all this is that a person's life situation provides everything necessary to achieve ideal growth. Our task is simply to employ our free will -- i.e. to properly and effectively use the opportunities that we have.
All the best to you in this and future lives.
http://www.aish.com/rabbi/ATR_browse.asp?s=reincarnation&f=tqak&offset=1