If one is born in a brāhmin family, he has got the opportunity to develop the brāhmin qualities. Similarly, if one is born in the family of a kṣatriya, he gets the opportunity of kṣatriya spirit. Similarly vaiśya. Cātur-varṇyaṁ mayā sṛṣṭaṁ guṇa-karma-vibhāgaśaḥ (BG 4.13). But the quality and actual action. Just like a boy is born of a medical practitioner, he has got greater chance of becoming, becoming a medical student, medical practitioners. But simply by getting birth as a son of medical practitioner is not sufficient. He has to take education. So cātur-varṇyaṁ mayā sṛṣṭaṁ guṇa-karma-vibhāgaśaḥ (BG 4.13). Kṛṣṇa does not say: "By birth." By acquiring the qualities and action. One must have the brahminical qualities and act as a brāhmin; then he'll be accepted as brāhmin.
So we are transmigrating according to our karma, different forms of life. The reason is, kāraṇaṁ guṇa-saṅgo 'sya sad-asad-janma-yoniṣu (BG 13.22). Kāraṇaṁ guṇa-saṅga, as you are associating with the guṇas, with the material qualities. So one has to increase his attachment for Kṛṣṇa by becoming nirguṇa. Everything is going on under the modes of material nature. Traiguṇya-viṣayā vedā nistraiguṇyo bhavārjuna. That is the instruction. One has to become above the . . . transcendental to these guṇas. And what is that process? The process is mad-āśraya. When you take shelter of Kṛṣṇa or you take shelter of Kṛṣṇa's pure devotee, then you become transcendental. Sa guṇān samatītyaitān brahma-bhūyāya kalpate. Who? Māṁ ca yo 'vyabhicāreṇa bhakti-yogena sevate (BG 14.26).