Proverbs 4:23 says to guard your heart "for it determines the course of your life" (NLT). Think about that. Whatever's in your heart is eventually going to show up in your everyday life. Whatever's inside will eventually find its way out, where anyone and everyone can see it.
That alone makes it extremely important to monitor the things we allow to shape our hearts. I don’t want something nasty, sinful, and selfish finding its way out and damaging my relationships with others, and I doubt you do either.
A large part of guarding your heart means learning how to control your thoughts, your words, your disposition and your general outlook. What you think usually comes out in what you say. What you say affects how you feel, and that shows up in your overall attitude.
In the course of everyday life, this is what determines how you handle your circumstances—whether you'll have peace or fall apart in a stressful situation. It governs how you respond to others, either with compassion and understanding or with judgment and arrogance—especially when you disagree with them!
You can try to keep your inner thoughts from altering your words and attitudes, but I find it’s much easier to have godly thoughts to begin with. Spend time in God’s presence, and let the Holy Spirit fill your heart with His goodness.