SB 6.1.51
tad etat sodasa-kalam
lingam sakti-trayam mahat
dhatte ’nusamsrtim pumsi
harsa-soka-bhayartidam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The subtle body is endowed with sixteen parts — the five knowledge-acquiring senses, the five working senses, the five objects of sense gratification, and the mind. This subtle body is an effect of the three modes of material nature. It is composed of insurmountably strong desires, and therefore it causes the living entity to transmigrate from one body to another in human life, animal life and life as a demigod. When the living entity gets the body of a demigod, he is certainly very jubilant, when he gets a human body he is always in lamentation, and when he gets the body of an animal, he is always afraid. In all conditions, however, he is actually miserable. His miserable condition is called samsrti, or transmigration in material life.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The sum and substance of material conditional life is explained in this verse. The living entity, the seventeenth element, is struggling alone, life after life. This struggle is called samsrti, or material conditional life. In Bhagavad-gita it is said that the force of material nature is insurmountably strong (daivi hy esa guna-mayi mama maya duratyaya). Material nature harasses the living entity in different bodies, but if the living entity surrenders to the Supreme Personality of Godhead, he becomes free from this entanglement, as confirmed in Bhagavad-gita (mam eva ye prapadyante mayam etam taranti te). Thus his life becomes successful.