Consider the surface of a lake, above which the sun is shining. If the surface of the lake is peaceful and tranquil, the sun will be reflected in this lake; and the more peaceful the lake, the more perfectly will it be reflected. If, on the contrary, the surface of the lake is agitated, undulating, then the image of the sun can not be reflected in it.
It is a bit like this with regard to our soul in relationship to God. The more our soul is peaceful and tranquil, the more God is reflected in it, the more His image expresses itself in us, the more His grace acts through us. On the other hand, if our soul is agitated and troubled, the grace of God is able to act only with much greater difficulty.~from Searching for and Maintaining Peace by Father Jacques Phillipe