The jiva-souls are eternal and have no beginning or end, they will never cease to be as Bhagavad Gita teaches.

The only reason we change material bodies is because in the material world nothing lasts forever, everything is in a constant state of decomposition except for the jiva-souls who are the passengers within these material vessels.

Bhagavad Gita - "For the soul there is never birth nor death, nor, having once been, does he ever cease to be. He is unborn, eternal, ever-existing, undying and primeval. He is not slain when the (material) body is slain." (BG Chapter 2 Text 12)

In most cases the cause of fall down to the material creation is free will.

Being a marginal living entity or jiva-soul means free will is included in their constitutional make up.

This means in Vaikuntha and Goloka-Vrindavana (The Kingdoms of God), the jiva-souls always have a choice.

Srila Prabhupada - "If you have no free will, then you are a stone. The stone has no free will. You want to be stone? Then you must have free will! But don't misuse your free will. But don't try to become stone either. That is not life." (August 5, 1976, New Mayapur France)

Sadly, less than 10% do choose to leave Krsna and Visnu and enter the dreaming material creation of Maha-Visnu.

The good thing is more than 90% of jiva-souls never choose to leave Vaikuntha or Goloka-Vrindavana Prabhupada has explained.

This fall down by some jiva-souls has nothing to do with the influence of Maya or the material energy because both do not exist in the spiritual planets.

However, free will does exists there and not only allows loving exchanges and reciprocation to exist with Krsna, but also allows the jiva-souls to reject Krsna if they choose.

A loving relationship with Kṛṣṇa and Visnu can only exist on a two-way street where each jiva-soul can also voluntarily contribute to the relationship.

Srila Prabhupada - "Love is reciprocal, voluntary, good exchange of feeling. Then there is love. Not by force; that is rape. Why one is called lover, another is called rape? In Bengali it is said if you catch one girl or boy and you demand you love me, you love me, is it love? You love me, otherwise I will kill you! Is that love? So Krsna does not want to become a lover like that, on the point of revolver. You love me, otherwise I shall kill you! That is not love; that is threatening." (July 8, 1976 in Washington, D.C)

Srila Prabhupāda – ''Genuine love means a relationship between "two persons", then there is exchange, then there is love. I must deal with you open-hearted, you must deal with me open-hearted, then there is love. So if you want to love Kṛṣṇa, God, then these things must be there." (August 9, 1976, Tehran)

Srila Prabhupada - ''Love means you take and you give also. Suppose if you love somebody and you simply take from him, but you don't give. Oh, do you think it is very good? No. It is not good. That is not love. That is exploitation. If I go on simply taking from you, and if I don't offer you anything, that is simply exploitation." (Lecture on BG Chapter 9 text 2-5, New York, Nov 23, 1966)

Srila Prabhupada - ''We have got the propensity to love. Love means somebody else. Love cannot be one or love cannot be executed by only one, there must be two. I love somebody; somebody loves me, there must be lover, there must be beloved, and the transaction, then love." (SB Canto 1 Chapter 2 text 6, Delhi, Nov 12, 1973)

Srila Prabhupada - ''The impersonalist philosophy is oneness, so how there be love with just one? Is it possible? Have you got any such experience that love means one? No. Love means two. There must be two, the lover and the beloved. Kṛṣṇa is already lover. He loves you so much that He's trying to get you back. That is Kṛṣṇa's attempt. "Please, My dear boy, or My dear friend, My dear servant." (SB Canto 2 Chapter 1 text 3, Paris, June 12, 1974)

Srila Prabhupada - "Surrender BY your intelligence but don't surrender your intelligence." (Letter to Bali Mardan 1974)

Love can never exist when only one is involved, the freedom must be there to voluntarily participate in a two-way reciprocal relationship with the Lord.

Even Krsna had to divide Himself into two just so He could experience loving exchanges with a women.

The word reciprocation only has meaning when there are two involved, NOT one.

Also, on very rare occasions, the jiva-souls can fall down due to some offence against a great devotee or devotees like the gatekeepers of Vaikuntha, Jaya and Vijaya who offended the 4 child like Kumaras by telling them they could not enter Vaikuntha.

As a result of their offence Visnu gave them two ways to enter the material creation, 7 times as devotees, or 3 times as demons.

Jaya and Vijaya could not stand the thought of being away from their master for seven lives, so they agreed to be born three times as the Lord's enemies. After fulfilling the curse, they could return to their immortal forms as His gatekeepers.  

The choice to leave Vaikuntha or Goloka Vrindavana is possible because Krsna allows love to be a two-way street, this means one can accept or reject Krsna if they choose to do so.

Srila Prabhupada – "So everyone can know that independence means one can use it properly, or one can misuse it. That is independence. If you make it one way only, that is not independence, that is force." (Los Angeles, June 23, 1975)

Srila Prabhupada – "You have got little independence therefore you can violate. Because you are part and parcel of God you have got independence, proportionately, therefore if he'd likes he can return. That independence has to be accepted, little independence. We CAN misuse that." (Conversation, Mayapur, February 19, 1976) 

Srila Prabhupada  - "Free will means that you can act wrongly. That is free will. Unless there is chance of doing wrong or right, there is no question of free will. Where is free will then? If I act only one sided, that means I have no free will. Because we act sometimes wrongly, that means free will."

Hayagriva - "A man may know better but still act wrongly."

Srila Prabhupada - "Yes, but that is free will. He misuses his. Just like a thief, he knows that his stealing, it is bad, but still he does it. That is free will. He cannot check his greediness, so in spite of his knowing that he is doing wrong thing—he will be punished, he knows; he has seen another thief, he was punished, he was put into prison— everything he knows, but still he steals. Why? Misuse of free will. Unless there is misuse of free will, there is no question of free will." (Philosophy Discussions with Srila Prabhupada – Rene Descartes)

There is only two causes that keep the jiva-souls continuingly taking birth in the material worlds-

1 - The result of accumulated past baggage of sinful and pious activity (Karma) from this and past lives forcing the conditioned in jiva-souls keep taking birth. 

2 - By not surrendering to Krsna's pure devotee and finding freedom from all karmic reactions and instead, continue to ignore Krsna by desiring to enjoy separate from Him or Visnu.

The jiva-souls are eternal, the only reason we change material bodies is because in the material world nothing lasts forever and material bodily vessels wear out, decay and die forcing the jiva-souls to take a new body.

This is called reincarnation.  

The material bodies we obtain at birth, are always on a constant journey in the temporary material creation where they are in a state of decay and decline, and eventually became food for maggots and worms.

Hence we experience birth, youth, middle age, old age and death.

This happens because all material bodies wear out, fade away, decompose and becomes unfit to house life (The jiva-souls).

Therefore the jiva-soul is eventually kicked out of the decomposing material vessel it is temporarily in and forced to take another material bodily vessel life after life.

And that re-birth could be any material body out of 8 million 400 thousand species of body.

The fact is to obtain the human bodily form is also very rare, most today are heading for rebirth in the animal kingdom as dogs, hogs, camels and asses..^..