Philippians 2:14-15

Do everything without murmuring or questioning so that you may prove yourselves to be blameless and guileless, innocent and uncontaminated, children of God without blemish in the midst of a (morally) crooked and (spiritually) perverted generation, among whom you are set as bright lights in the world (of darkness). 

There are times when circumstances are frustrating, and all we want to do is verbalize our frustration, thinking it will make us feel better. God instructs us to hold our tongues and not complain amid trials. Why? Because God is the answer to our problems and can help us when we seek Him over the matter.