Sin is a spiritual and moral malignancy. Left unchecked, it can spread through our entire inner being and contaminate every area of our lives. Even worse, it often will "metastasize" from us into the lives of other believers around us. None of us lives on a spiritual or social island. Our attitudes, words and actions, and oftentimes even our private unspoken thoughts, tend to have an effect on those around us.
Paul must have had this concept in mind when he wrote, "Let no corrupting talk come out of your mouths, but only such as is good for building up, as fits the occasion, that it may give grace to those who hear" (Ephesians 4:29). Our speech, whether it is about others or to others, tends to tear down or build up. It either corrupts the minds of our hearers, or it gives grace to them. Such is the power of our words. If I gossip, I both tear down another person and corrupt the mind of my listener. If I complain about the difficult circumstances of my life, I impugn the sovereignty and goodness of God and tempt my listener to do the same. In this way, my sin "metastasizes" into the heart of another person.
From Respectable Sins by Jerry Bridges, page 23.