The conditioned life of a living being is caused by his revolting against the Lord. There are men called deva, or godly living beings, and there are men called asuras, or demons, who are against the authority of the Supreme Lord. In the Bhagavad-gītā (BG Sixteenth Chapter) a vivid description of the asuras is given in which it is said that the asuras are put into lower and lower states of ignorance life after life and so sink to the lower animal forms and have no information of the Absolute Truth, the Personality of Godhead.
-Shrimad Bhagvatam Canto 1 Chapter 2 Verse 16, Translation and Purport

यस्य वाचा प्रजा: सर्वा गावस्तन्त्येव यन्त्रिता: ।
हरन्ति बलिमायत्तास्तस्मै मुख्याय ते नम: ॥ ८ ॥
All the living entities within the universe are conducted by the Vedic directions, as a bull is directed by the rope attached to its nose. No one can violate the rules laid down in the Vedic literatures. To the chief person, who has contributed the Vedas, we offer our respect!
-Shrimad Bhagvatam Canto 3 Chapter 15 Verse 18