“God does not prevent sin, because even from the very abuse man makes of the liberty with which he is endowed, God knows how to bring forth good and to make his mercy or his justice become more and more resplendent.” St. Pius X 

God could prevent sin. To do so is to take away the freedom that he gives us to choose to love or not to love. He allows sin so that he can show the depth of his love through his unconditional mercy or the consequence to unrepentant sin through his righteous justice.  This is why God is God. His ways are not our ways. Living in his way is to live in love. But when we sin, we can return to the Lord for his mercy is everlasting. Each time we do, we are proclaiming the greatness of our God in spite of our weakness.