Final answer:
Augustine sinned frequently due to his belief in free will and the conflict between good and sinfulness. He believed that while divine grace could ultimately resolve this conflict, individuals still had the power to choose whether to sin.
Step-by-step explanation:
The main reason Augustine sinned so frequently is because he believed in the concept of free will and that it was within his power to choose between good and evil. According to Augustine, there was a conflict between two rival wills - one that willed the good and one that desired sinfulness. While divine grace could ultimately resolve this conflict, Augustine believed that individuals still had the power to choose whether to sin.