Why are we not to trust our hearts?

Many people say for us to follow our hearts and that will guide us into good decisions.  However, the Bible tells us that the heart is deceitful and desperately wicked (Jeremiah 17:9).  It also states that the heart brings forth evil thoughts, murders, adulteries, fornications, thefts, false witness, and blasphemy and that all these things defile us (Matthew 15:19-20).  Simply just following our heart will not get us anywhere, and we should not trust ourselves to make good choices on our own.  We need God to direct our hearts in the way to go and make sure we follow Him always (Proverbs 3:5-6; Psalm 73:26).