It is stated in the Bhagavad-gītā, apareyam itas tu viddhi me prakṛtiṁ parām (BG 7.5). The living entities, they are prakṛti. And they are trying to be puruṣa, enjoyer. This is material world. Therefore they are attached to the three guṇas, because they want to enjoy this material world, artificially trying to become puruṣa. Puruṣa means enjoyer. As such, they are attached to this material world. And material world means guṇa, tri-guṇa. And according to attachment, they're getting different types of bodies.
That is also stated in the Bhagavad-gītā, kāraṇaṁ guṇa-saṅgaḥ asya sad-asad-yoni-janmasu (BG 13.22). Why there are different types of bodies? Why one living entity is dog and why one living entity is first-class brāhmaṇa? Because kāraṇaṁ guṇa-saṅgaḥ asya. The one who has got a learned Brāhmiṇ's body, that means he has associated with the modes of nature goodness. Satyaṁ śamo damas titikṣā. This is elevation. If one is in the modes of goodness, then he can understand gradually what is God, what is Kṛṣṇa. Otherwise, in the modes of passion and ignorance, nobody can understand what is God. Not only that. Even from material prosperity, if one is situated in the modes of goodness, then he can be elevated to the higher planetary system—Svargaloka, Janaloka, Maharloka, Tapoloka, Brahmaloka, Satyaloka. Ūrdhvaṁ gacchanti sattva-sthāḥ (BG 14.18). So these are needed. So anyway, even if you go to the Brahmaloka, that is not also perfection. Ābrahma-bhuvanāl lokāḥ punar āvartino 'rjuna (BG 8.16). There is no benefit. Because there also the four principles of miserable condition, janma-mṛtyu-jarā-vyādhi (BG 13.9), they are there, even in the Brahmaloka. Brahmā also dies. Brahmā also takes birth. You know it. Brahmā, he also took birth, from the lotus flower generated from the navel of Viṣṇu. So there was birth. And when Brahmā will die, whole material cosmic manifestation will be finished. So he has also birth and death and old age and disease. And the small ant or insect, it has also the same disease, old age, birth and death. So one has to become free from this bondage, because we are eternal.