Because the conditioned soul sometimes desires to forget God, by His grace, the Lord provides the means for it to do so, almost perpetually, life after life.
The more a person distances himself from God, the more he rejects His precepts, laws and commandments, and the more his mind becomes clouded and confused, and the more he is carried away by hatred and violence. He does not know that in reality it is God who sustains this world, who directs it, and that he can never do anything about it.
It is the activities aimed at the satisfaction of the senses and whose sole purpose is to please the mind and the senses, which are the cause of the bondage to matter. As long as the soul indulges in these selfish actions, it will not cease to reincarnate from one species to another.
Krishna, God, the Supreme Person says:
When one considers sense gratification as the goal of his life, he engages in material life to the point of insanity and indulges in all sorts of sinful activities. He does not know that it is because of his past misdeeds that he has already been given a material body, which, despite its transitory nature, is the cause of his suffering. The truth is that the separate being should never have taken on this carnal envelope, but it has been given to him for the satisfaction of his senses. Therefore, I do not believe that it is fitting for an intelligent man to become entangled again in material activities which would force him to put on bodies, life after life. As long as the living being does not enquire into the spiritual values of existence, he must experience defeat and the evils of ignorance.
Whether it is virtue or sin, karma bears its fruits, and if a person is involved in any form of karma, his mind is said to be “tainted” with the desire to enjoy the fruits of action.
As long as the mind remains impure, the consciousness remains obscured, and as long as one follows the path of self-interested action, one must put on a material body. When the living being is under the influence of ignorance, he cannot understand the nature of the separate soul [the individual spirit soul separate from God] and that of the Supreme Soul, his mind is then yoked with self-interested action.
Therefore, as long as he has no love for God, he will certainly not be exempted from putting on material bodies.
The Lord adds: Under the influence of the three gunas [the three attributes and modes of influence of material nature; virtue, passion and ignorance], the soul misled by the false ego [believing itself to be the body and seeking to dominate matter] believes that it is the author of its acts, whereas in reality they are performed by material nature.