Prakṛti, nature, is doing everything. I am desiring something. That means I am contacting, contacting the certain type of the modes of material nature. This is also explained in the Bhagavad-gītā: kāraṇaṁ guṇa-saṅgaḥ asya sad-asad-janma-yoniṣu (BG 13.22). Sad-asad-janma-yoni. Yoni means the source of birth. Mother is called yoni, and father is called bīja. Yathā yoni yathā bījam. According to bījam and according to yoni, we get body.
So why you are getting different types of body? Kāraṇaṁ guṇa-saṅgaḥ asya. He is contacting a particular type of the modes of material nature. There are three modes of material nature: sattva-guṇa, rajo-guṇa, tamo-guṇa. So if I am associating with sattva-guṇa, then next life I'll get a sattva-guṇa body. Ūrdhvaṁ gacchanti sattva-sthā. (BG 14.18) Sattva-guṇa body means higher standard of life. So similarly, if I am associating with the modes of passion, then I'll get a body, next life, passionate.
If I am associating with ignorance, tamo-guṇa . . . jghanya-guṇa-vṛtti-sthā adho gacchanti tāmasāḥ. Tāmasāḥ. Tama-vṛtti. The jaghanya, the lowest. There are so many varieties of life. So those who are associating with tamo-guṇa—drinking, flesh-eating, smoking, illicit sex, gambling . . . these are the association of tamo-guṇa. Yatra pāpāś catur-vidhā (SB 1.17.38), where there are four kinds of sinful activities. So intoxication is sinful activities. That is tamo-guṇa, associating with tamo-guṇa. So jaghanya-guṇa-vṛtti-sthā adho gacchanti tāmasāḥ (BG 14.18). Those who are associating with tamo-guṇa, their vṛtti, their habits, are very abominable. Jaghanya. Therefore, adho gacchanti: they go downwards.
In this way, everyone is enjoying a particular type of rasa, material rasa . . . and nature is offering him a particular type of body. Deha-yogena dehinam (SB 7.6.3). We can enjoy a particular type of rasa . . . just like the hogs: they are relishing the rasa of stool very nicely. You give them nice food, they will not take. They'll prefer to taste the stool. Why? Because he has been offered a particular type of body. And this particular type of body has been offered to him, kāraṇaṁ guṇa-saṅgaḥ asya . . . (BG 13.22) He has associated with jaghanya, tamo-guṇa, abominable tamo-guṇa; therefore he has developed a body of a hog, and tasting the juice of stool. This is the way of transmigration of the soul.
Actually, it is the living spirit soul, either hog or dog or a brāhmin or a caṇḍāla or a cow. This is simply a covering, or . . . just like we cover ourself in dream. That is subtle covering. Similarly, this is also covering. In the Vedas it is said, asaṅgo hy ayaṁ puruṣaḥ (Bṛhad-āraṇyaka Upaniṣad 4.3.16). Actually, the living entity, the spirit soul, is neither hog, neither dog, nor brāhmiṇ, nor this, nor that. It is simply a covering. But this covering is developed by our association with the particular type of modes of nature. Kāraṇaṁ guṇa-saṅgaḥ asya sad-asad-janma-yoniṣu (BG 13.22).