Let not your heart be troubled; you believe in God, believe also in Me. John 14:1
To fret means to chafe, to be irritated, to be uneasy, to be troubled and bothered. It is just the opposite of peaceful, trustful rest. Jesus has promised us rest to our souls, and we may have this rest. We can not have it, however, if we give place to worrying and fretting. God's purpose for us is that we shall have calmness and soul-quietness, even in the midst of tribulation. He has said, "My peace I give unto you." He followed this by saying, "Let not your heart be troubled, neither let it be fearful" (John 14: 27).

It is not God's will that we be continually worrying. This world is full of things that are not as they ought to be, and if we are to be happy and peaceful we must adjust ourselves to circumstances and learn to be happy in spite of the things that are displeasing to us, that are not as they ought to be. We can never be amidst ideal conditions in this world.

Fretting is like sand in a bearing; it is likely to make all sorts of trouble. It will use up the energy that we ought to be using for something else; it will keep our physical and spiritual nerves on edge; it will spoil the tranquility of our lives; it will mar our peace and take the sweetness out of our devotions.

(An excerpt from a sermon by W Charles Naylor)