Musar על כתובות 118:15

And the Sages also said, "If it were not for the fact that a man forgets the Torah if he does not study it constantly, he would learn all of the Torah and after that he would go idle, and idleness leads a person to boredom and to sin (Kethuboth 5:5; Aboth de-Rabbi Nathan chap. 11). Therefore, it was decreed that the Torah is easily forgotten, so that man would occupy himself with Torah all of his days, and thus not come to sin.