This to me is a divine command; the same as "Thou shalt not steal." Now let us get to the definition of fretting. One good definition is, "Made rough on the surface." "Rubbed, or worn away"; and a peevish, irrational, fault-finding person not only wears himself out, but is very wearing to others. To fret is to be in a state of vexation, and in this psalm we are not only told not to fret because of evildoers, but not to fret "in anywise." It is injurious, and God does not want us to hurt ourselves.
A physician will tell you that a fit of anger is more injurious to the system than a fever, and a fretful disposition is not conductive of a healthy body; and you know the rules are apt to work both ways, and the next step down from fretting is crossness, and that amounts to anger. Let us settle this matter, and be obedient to the command, "Fret not." MARGARET BOTTOME
Overheard in an Orchard
Said the Robin to the Sparrow:
"I should really like to know
Why these anxious human beings
Rush about and worry so."
Said the Sparrow to the Robin:
"Friend, I think that it must be
That they have no Heavenly Father
Such as cares for you and me."
Exerpt from Streams in the Desert by L.B. Cowman
"Therefore encourage one another and build one another up, just as you are doing."