Harshness is a hammer. Gentleness is a mallet. Both hammers and mallets are used for moving other objects into their proper place. But the way they work and the effect they have on the object is quite different.
Hammers have a metal head with a concentrated point of force, causing objects to be moved into place without concern for the indentations made on their surfaces. You use hammers on objects you don’t mind getting scuffed up, such as nailheads. But mallets have a wooden head with a broader, more distributed point of force, causing objects to be moved into place without damage to the surface. You use mallets on objects you want to stay beautiful, such as wood trim.
Harshness is a manner of relating to others with words and actions that concentrate exclusively on what is wrong with them, resulting in their discouragement. Notice the trajectory of that last sentence. Harshness begins with a way of seeing people through a judgmental eye and then comes out in words that have an effect that is negative rather than uplifting. Those words usually focus attention on what is irritating or disappointing in someone else, and that creates discouragement and bitterness.
Scripture says that harshness is unlike God. It is the opposite of gentleness, which is a characteristic of love (Gal. 5:22–24). Gentleness is a way of interacting with others that accounts for the likely effect that your approach will have on them. It has the other person’s edification at heart and adjusts accordingly. Gentleness is like a mallet that moves wood trim into place with a high priority on not damaging the trim.