“This vast universe is a wheel. Upon it are all creatures that are subject to birth, death, and rebirth. Round and round it turns, and never stops. It is the wheel of Brahman. As long as the individual self thinks it is separate from Brahman, it revolves upon the wheel in bondage to the laws of birth, death, and rebirth. But when through the grace of Brahma it realizes its identity with him, it revolves upon the wheel no longer. It achieves immortality.” —Svetasvatara Upanishad (Prabhavananda), 118
If from life to life we continually protect and nourish our desires, fears, and resentments, and deal with life through anger, jealousy, vanity, and pride, then we build tremendous formations in our minds that exert control over us continually. Then we die... and are reborn, propelled by the contents of our psyche, in accordance to the weight in our minds.
Thus, we return to a new body, whose every feature and circumstance is determined by our previous actions. Our parents, our social status, country, language, religion, education—everything—is a result of the cause and effect we established in our previous existences. Everything we are now is a result of what we did before.
We return in order to experience the effects of our previous actions—to exhaust our karma. Unfortunately, we tend to add to our karma more than pay it, thus from existence to existence we tend to deepen our debts and mistakes, rather than reducing them.
Just like a drop of water rolling to the edge of a leaf, at a certain moment the water hangs on the edge, then falls. So it us with us.
Return can only happen if our karmic debts are not too deep. Once they reach a certain critical mass, the droplet falls: we receive no more physical bodies. Nature takes our mind in order to purify it. Most religions call this process hell.