(Prabhupada's lecture - Śrīmad-Bhāgavatam 1.15.47-48 - Los-Angeles, December 25, 1973)
That is explained in the Bhagavad-gītā. Kāraṇaṁ guṇa-saṅgo 'sya. As you have infected yourself with a particular type of modes of nature, you will get a body. That's all. If you have infected to get the body of a dog, you must be infected. People who are keeping dog, that means he is being infected by the germs of dog. Next life he will become a dog. He is infected. What he will think at the time of death? If he has got a beloved dog, he will think of... Somebody... In your country, they bequeath great, I mean to say, wealth, for the dog. That means at the time of death, at the time of death he is thinking of his beloved dog. So he must be a dog. So all his wealth goes to the dog and he goes to become a dog.
This science is unknown to the rascals. Yaṁ yaṁ vāpi smaran bhāvaṁ tyajaty ante kalevaram [Bg. 8.6]. At the time of death, your position of the mind will create the next body. This is the science. So if you create your the position of the mind, Kṛṣṇa conscious, then you will get a body like Kṛṣṇa, sac-cid-ānanda-vigraha [Bs. 5.1]. This is Kṛṣṇa consciousness movement, that "Why you shall love dog and cats? Love God, Kṛṣṇa, and you get the next body like Kṛṣṇa." Sac-cid-ānanda-vigraha. Anādir ādir govindaḥ sarva-kāraṇa-kāraṇam [Bs. 5.1]. This is Kṛṣṇa consciousness movement, how to think of Kṛṣṇa at the time of death. Then your life is successful.