We are primarily born because we have unfulfilled desires from our past life. We attempted to know the Truth but we did not finish the job. We still have passions, greed, lust, and desire for name and fame. So as long as a person has desires and he wishes to have something in return, there is a need to have a body to do or perform these desires.

It is the greatness of God that he gives us the instruments -- the body, mind  and intellect -- to fulfill our desires. If we don't have desires, we will not need the body and we will not need birth either. When all these desires come to an end, the cycle of birth and death also come to an end.

--Swami Radhanandaji