There are three main reasons sentient beings perpetuate in samsara: first, clinging to impermanence as permanence; second, seeing activities that are inherently unsatisfactory as joyful; third, mistaking a non-existent self for a truly existent self. With these three forms of attachment, good and bad karma is created. With good karma, beings take rebirth in the upper realms of the gods and human beings; with bad karma, beings take rebirth in the lower realms such as hell, unable to transcend samsara and attain liberation.

~Decpited from THE FOUR SEALS OF DHARMA - SUMMATION