The overalls of a mechanic can sometimes become so stained and blotted with grease and oil, that beneath the layers of filth the colour of the overall itself becomes lost and almost impossible to determine! Due to the overalls already being so filthy, it won’t bother the mechanic in the least if a few more drops of oil have to splash onto him.
On the contrary, a person immaculately dressed in pristine white clothing would probably be unable to tolerate even a single drop splashing onto and staining his clothing. If his clothing does become marred by a stain, he will immediately whip out the ‘Vanish’ and try to remove it.
Similar is the case of a person whose heart is covered in layers of muck and filth which accumulate through continuous sinning and the lack of repentance. Due to the heart being so overwhelmed by this flood of filth, additional sins will not irk the conscience in the least. The heart becomes so dead that the person can now freely intermingle with people of the opposite sex and claim “there is nothing in my heart” because this dead heart really does not feel anything anymore.
A person who keeps his heart pure and free of sin, however, will find the blot and stain of even a single sin too much to bear. If he still does somehow fall into sin, his heart will not let him rest until he has repented and made amends, thereby removing the stain.