Musar על ברכות 119:31

Therefore, they will choose only to maximize these means and to be stringent in all of their details, finding no rest or peace due to worry lest they possibly lack what will bring them to the perfection that they desire. This is what King Shlomo, peace be unto him, said: "fortunate is the man that fears always" (Mishlei 28:14), which our Sages explained (Berachot 60a) refers to matters of Torah.

The sages already denigrated this trait and attributed it to the sinners, with many verses to support them such as "the sinners in Zion are afraid; trembling has seized the flatterers." (Isaiah 33:14). One of the great sages upon noticing his disciple afraid said to him: "you are a sinner" (Berachot 60a).

This is why it is called "fear of sin", for its primary matter is fear of sin (not G-d), that it not enter and mix in his deeds due to some negligence or laxness or due to forgetfulness for whatever reason. On this it was said: "Fortunate is the man that fears always" (Mishlei 28:14), which our sages of blessed memory explained: "that verse refers to words of Torah" (Berachot 60a). For even when one does not see a stumbling block before his eyes, his heart must feel dread within lest there be one hidden at his feet and he did not guard [from it].