The living entity receives different types of bodies according to karma. One living entity may receive a cat body, another a dog body, and so forth. Why are there so many different bodies? Why not one kind of body? The answer to this is also given in Bhagavad-gita (13.22):
karanam guna-sango ‘sya sad-asad-yoni-janmasu
“It is due to his association with the modes of material nature. Thus he meets with good and evil among various species.” Because the soul within the body associates with the three modes of material nature (goodness, passion and ignorance), he receives different types of bodies. One doesn’t have to aspire for his next body; one need only rest assured that it will be a different body. On the other hand, Kṛṣṇa does not say what kind of body one will be awarded. That depends on qualification. If one associates with the mode of goodness, he is elevated to the higher planetary systems. If he associates with the mode of passion, he remains here. And if one associates with the mode of ignorance and darkness, he goes down to lower life forms-animals, trees and plants. This is the proclamation of Sri Kṛṣṇa in Bhagavad-gita (14.18):
urdhvam gacchanti sattva-stha madhye tisthanti rajasah
jaghanya-guna-vrtti-stha adho gacchanti tamasah
“Those situated in the mode of goodness gradually go upward to the higher planets; those in the mode of passion live on the earthly planets; and those in the mode of ignorance go down to the hellish worlds.” There are 8,400,000 species of life, and all of these arise from one’s association with the modes of nature (karanam guna-sango ‘sya). And, according to the body, one undergoes distress and happiness. One cannot expect a dog to enjoy the same happiness that a king or rich man enjoys. Whether one enjoys this or that happiness or suffers this or that distress, both distress and happiness are due to the material body.
Yoga means transcending the distress or happiness of the material body. If we connect ourselves with Kṛṣṇa through the supreme yoga, we can get rid of material happiness and distress arising from the body. Reconnecting with Kṛṣṇa is called bhakti-yoga, and Kṛṣṇa comes to instruct us in this supreme yoga. In essence, He says, “Just revive your connection with Me, you rascal. Give up all these manufactured yogas and religions and just surrender unto Me.” That is Kṛṣṇa’s instruction, and Kṛṣṇa’s representative, the incarnation or the guru, says the same thing. Although Kapiladeva is an incarnation of Kṛṣṇa, He acts as the representative of Kṛṣṇa, the guru. If we just accept the principle of surrender unto Kṛṣṇa, we will become actually transcendental to so-called material happiness. We should not be captivated by material happiness or aggrieved by material distress. These are causes for bondage. Material happiness is not actual happiness. It is actually distress. We try to be happy by obtaining money, but money is not very easily obtained, and we have to undergo a great deal of distress to get it. However, we accept this distress with the hope of getting some false happiness. If we purify our senses, on the other hand, we can come to the spiritual platform. Real happiness lies in engaging our senses to satisfy the senses of Kṛṣṇa. In this way our senses are spiritualized, and this is called adhyatmika-yoga or bhakti-yoga.
Source: A.C. Bhaktivedanta Swami Prabhupada (2007 edition), “Teachings of Lord Kapila, The Son of Devahuti”, Page 89, 90 & 91