Peace be upon you. What is the reason for not regretting sins?

Repentance from sins is an immediate obligation. Negligence of Allah, His remembrance, and the recitation of His Book causes hardness of the heart and familiarity with sin. Among the greatest aids to repentance are: remembering the greatness of Allah, remembering death and standing before Him, contemplating the evil consequences of sins, and shunning bad places and companions while seeking out good places and companions.