SB 3.30.4

jantur vai bhava etasmin
 yāṁ yāṁ yonim anuvrajet
tasyāṁ tasyāṁ sa labhate
 nirvṛtiṁ na virajyate
Synonyms: 
jantuḥ — the living entity; vai — certainly; bhave — in worldly existence; etasmin — this; yām yām — whatever; yonim — species; anuvrajet — he may obtain; tasyām tasyām — in that; saḥ — he; labhate — achieves; nirvṛtim — satisfaction; na — not; virajyate — is averse.
Translation: 
The living entity, in whatever species of life he appears, finds a particular type of satisfaction in that species, and he is never averse to being situated in such a condition.
Purport: 

The satisfaction of the living entity in a particular type of body, even if it is most abominable, is called illusion. A man in a higher position may feel dissatisfaction with the standard of life of a lower-grade man, but the lower-grade man is satisfied in that position because of the spell of māyā, the external energy. Māyā has two phases of activities. One is called prakṣepātmikā, and the other is called āvaraṇātmikā. Āvaraṇātmikā means “covering,” and prakṣepātmikā means “pulling down.” In any condition of life, the materialistic person or animal will be satisfied because his knowledge is covered by the influence of māyā. In the lower grade or lower species of life, the development of consciousness is so poor that one cannot understand whether he is happy or distressed. This is called āvaraṇātmikā. Even a hog, who lives by eating stool, finds himself happy, although a person in a higher mode of life sees that the hog is eating stool. How abominable that life is!