The book Ecclesiastes is a Sapiential Book of the Old Testament, the Book begins with the words of its author Kohelet, son of David, king of Jerusalem. Kohelet expresses in his book that he dedicated "to investigate and to explore with wisdom all that is done under heaven," Ecclesiastes 1:13.
The tradition identifies the "doctor or master of wisdom" Kohelet with the King Solomon.
For the author of this work, not only wisdom is important: "Wisdom is worth as much as an inheritance", Ecclesiastes 7:11, but also the predisposition to acquire it:

"Happiness is for those who fear God
because they feel fear in his presence" Ecclesiastes 8:12

In the Ecclesiastes the Fear of God represents the predisposition to acquire the "science of God", the wisdom: the fool and the impious despise it. The Fear of God is one of the 7 gifts of the Holy Spirit according to Thomas Aquinas and is defined as constancy, firmness, equanimity, stability, devotion.
Constancy predisposes us to acquire wisdom:

The fear of Jehovah is the beginning of the wisdom. Proverbs 1:7

According to the Ecclesiastes the Fear of God is the base for the most excellents virtues of man, the wise man achieves nothing without Fear.
Thomas Aquinas, in his Summa Theologica Book tells us that there is a "opinated happiness" and true happiness.
The opposite of happiness is suffering. The happiness as an end itself is an opinion, a form of error, because to be happy we have to minimize the suffering. This is achieved in one way: having constancy in excellence, having constancy in improving ourselves.
Constancy, the firmness is the base of the spiritual life, the blessed life. Let us remember that eternal life are blessings that God gives us as an advance of life in glory, and among these blessings is the happiness of the heart, the happiness associated with the contemplative activity. And who attain eternal life? This is explained in the parable of the sower: those who receive wisdom, retain it and put it into practice with constancy, firmness. The spiritual gift of Fear of God or serenity (constancy, firmness, devotion, equanimity) is the way to the spiritual enlightenment and true happiness.