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

प्रायेणाल्पायुष: सभ्य कलावस्मिन् युगे जना: ।
मन्दा: सुमन्दमतयो मन्दभाग्या ह्युपद्रुता: ॥ १० ॥
O learned one, in this iron Age of Kali men almost always have but short lives. They are quarrelsome, lazy, misguided, unlucky and, above all, always disturbed.
-SB 1.1.10