SB 9.13.10
deham navarurutse ’ham
duhkha-soka-bhayavaham
sarvatrasya yato mrtyur
matsyanam udake yatha
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
I do not wish to accept a material body, for such a body is the source of all distress, lamentation and fear, everywhere in the universe, just as it is for a fish in the water, which lives always in anxiety because of fear of death.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The material body, whether in the higher or lower planetary system, is destined to die. In the lower planetary system or lower species of life one may die soon, and in the higher planets or higher species one may live for a long, long time, but death is inevitable. This fact should be understood. In the human form of life one should take the opportunity to put an end to birth, death, old age and disease by performing tapasya. This is the aim of human civilization: to stop the repetition of birth and death, which is called mrtyu-samsara-vartmani. This can be done only when one is Krsna conscious, or has achieved the service of the lotus feet of the Lord. Otherwise one must rot in this material world and accept a material body subject to birth, death, old age and disease.
The example given here is that water is a very nice place for a fish, but the fish is never free from anxiety about death, since big fish are always eager to eat the small fish. Phalguni tatra mahatam: all living entities are eaten by bigger living entities. This is the way of material nature.
ahastani sahastanam
apadani catus-padam
phalguni tatra mahatam
jivo jivasya jivanam
“Those who are devoid of hands are prey for those who have hands; those devoid of legs are prey for the four-legged. The weak are the subsistence of the strong, and the general rule holds that one living being is food for another.” (Bhag. 1.13.47) The Supreme Personality of Godhead has created the material world in such a way that one living entity is food for another. Thus there is a struggle for existence, but although we speak of survival of the fittest, no one can escape death without becoming a devotee of the Lord. Harim vina naiva srtim taranti: one cannot escape the cycle of birth and death without becoming a devotee. This is also confirmed in Bhagavad-gita (9.3). Aprapya mam nivartante mrtyu-samsara-vartmani. One who does not attain shelter at the lotus feet of Krsna must certainly wander up and down within the cycle of birth and death.