"... Christ also suffered for you, leaving you an example for you to follow in His steps, who committed no sin, nor was any deceit found in His mouth; and while being reviled, He did not revile in return; while suffering, He uttered no threats, but kept entrusting Himself to Him who judges righteously (1 Pet 2:21-23)."
Jesus is our example even in speech. He did not revile even when he was being reviled. That is why James says,
" Do not speak against one another, brethren. He who speaks against a brother or judges his brother, speaks against the law ... (Jas 4:11)."
Impure speech is a characteristic of the old person, and God's desire is for the death of the old person. It is the new person that bears the image of God, not the old.
"... put them all aside: anger, wrath, malice, slander, and abusive speech from your mouth. Do not lie to one another, since you laid aside the old self with its evil practices, and have put on the new self who is being renewed to a true knowledge according to the image of the One who created him--(Col 3:8-10)"
Notice how many characteristics of the defiled old self have to do with speech. Why is speech so important? Why does God make such a big deal out of it? Words are an indication of what is in the heart.
"Either make the tree good and its fruit good, or make the tree bad and its fruit bad; the tree is known by its fruit. You brood of vipers, how can you, being evil, speak what is good? For the mouth speaks out of that which fills the heart. ... But I tell you that every careless word that people speak, they shall give an accounting for it in the day of judgment. For by your words you will be justified, and by your words you will be condemned (Matt 12:33-37)."
So words are extremely important. If unwholesome words proceed from our mouth, then we have a heart problem. That is why Jesus says to make the tree good, then the fruit of our lips will be good.