"The heart is deceitful above all things, and desperately wicked: who can know it?" Jeremiah 17:9
First the heart is deceitful to you. When your flesh desires something or someone that is not from God, your heart will go along with your flesh unless you are disciplined in lifestyle. Your flesh screams with desire and your mind rationalizes your actions to fulfill your desires. Those desires from God within the parameters of living a life in alignment with God are good and should be nurtured. But those desires for that which has not been ordained by God will tear you apart and ruin your chances for a peaceful and contented life.
God speaks clearly to us through the Bible and clearly to us in our heart to empower us to overcome, but our heart can be hardened and it can choose a alternate way from the way of God. Why is this important? Because those who make up their own private belief system are not advancing the kingdom nor are they living their life in a way that glorifies God, nor are they leading their children or others in the right direction.
It matters what you think and how you process the events of the day. Your future relies on good judgement and others who are in desperate need of a right word or a good example are counting on you to provide them with it. Being a Christian is about doing what God says to do, quite often it is not what you feel like doing and even more often it is not going to satisfy some temporal flesh desire. Follow your heart is a good saying if your heart has been purchased by God and you are lending you rights and will to God for use. But following your heart if you have not come to the place where you realize that your actions and decisions need to be guided by prayer and the Word of God, well you just can't count on it.
It's true, you just can't count on the heart of those who are more interested in building themselves up and advancing their own cause than that of God's.
You just can't count on the heart of those who are willing to live outside of the will of God under the pretence that they aren't hurting anybody.
You just can't count on the heart of the believer that lies and deceives and tries to get others to go along with them in order to build themselves up or make themselves look good.
You just can't count on the heart of those who look teary eyed into you eyes in hopes that you will continue to support them in their illegal/illicit/immoral lifestyle.
You just can't count on the heart of those who say one thing and do another.
You can count on the heart of God to lead you and guide you as you yield your will to His. Can a man's heart be changed so that you can count on him? Yes, but only by the grace of God and only in due time. Maybe you need a change of heart. Maybe you can't trust your own heart because it has deceived you so many times. Maybe you are stricken with a habit that is killing you and destroying your family and you need a change of heart. You can count on God. Just be humble and let God know that you have been trying to do it on your own and you need His help. God will be there and is there now. God can change a heart.