Gregory the Dialogist

Thou oughtest not, then, in this life to have security, whereby thou mayest be rendered careless. For it is written, "Happy is the man that is always afraid." And again it is written, "Serve the Lord in fear, and rejoice unto him with trembling." In short, then, it must needs be that in the time of this life trembling possess your soul, to the end that it may hereafter rejoice without end through the joy of security.

Register of Epistles, Book 7, Epistle XXV ↗

Bede

"Blessed is the man who is always fearful," etc. The wrath of the upcoming judgment will then be so much stricter as it is now scarcely feared amidst sins. Hence, it is said elsewhere, "In the day of prosperity, do not be unmindful of adversity; and in the day of adversity, do not be unmindful of prosperity" (Ecclesiasticus 11). Thus, both should be joined so that one always supports the other.

Commentary on Proverbs ↗

Bonaventure

In the same manner, there are four acts of justice: to do good, to flee evil, to beware of prosperity, and to withstand misfortune. Of the third: "Happy the man who is always on his guard" when things go well with him. Hence the just is warned not to fall. If he does fall, he is advised to rise again. For "the just man falls seven times and rises again."

Collations on the Hexaemeron, Collation 18 ↗