Musar sur Les Proverbes 4:19

Thus the simpletons walk confidently, fall and are lost without having felt any prior fear. This is what scripture refers to: "the way of the wicked is as darkness; they know not at what they stumble" (Prov. 4:19), and "the clever one foresees the evil and hides himself, but the foolish commits transgression and is punished" (Prov. 22:3), and "[a wise one fears and departs from evil], but the fool transgresses and feels confident" (Prov 14:16). For they feel as secure as an edifice, and they fall before having any knowledge whatsoever of the stumbling block.

For after this has become clear to them and likewise after it has become clear to them that the means to perfection are good deeds and traits, they will certainly never consent to diminish these means or be lenient in them. Since, it has already become clear to them that if they diminish in these means or are weak in these means, not employing the full force necessary, they will not attain the true perfection. Rather, it will be reduced in proportion to their reduction in exerting themselves to the necessary extent, leaving them lacking in perfection which is a great calamity and great evil to them.

The pinnacle of this level called "fear of sin", one of the greatest levels, is when a man is constantly afraid and worried lest he have in his hand some trace of sin which obstructs him from the perfection that he is under duty to strive for.

Regarding this our Sages of blessed memory said: "this teaches each person is burned from the Chupa (canopy) of his fellow" (Bava Basra 75a). This [burning] does not refer to jealousy which falls only to people lacking in understanding as I will explain with G-d's help. Rather, it is due to seeing oneself lacking from the perfection that he was capable of attaining just as his fellow had attained it.