"And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually. And it repented the Lord that he had made man on the earth, and it grieved him at his heart. And the Lord said, I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth me that I have made them. But Noah found grace in the eyes of the Lord." -- Genesis 6:5-8
The story of Noah tells us that many people today are being destroyed for the simple reason that their hearts are wrong. There were three heart issues concerning the people of Noah's day that displeased God: wickedness, evil imaginations, and evil thinking. But Noah had a right heart and found favor in the eyes of the Lord.
We can not imagine how many areas of our lives would be straightened out if we would just get our hearts in the right place. Our hearts may not be filled with the exact same evil thoughts and imaginations of the people in Noah's day, but a bad attitude or wrong thinking can also be labeled evil imaginations and evil thinking. If we have a bad attitude and "stinking thinking" we are going nowhere in life.
This is why we must guard our heart -- out of our heart flows the issues of our life. Our problem is that if we let garbage in, garbage will come back out. Allowing negative, evil thinking in our hearts cannot produce a life that glorifies God. We have to be careful not only about our actions but also about our imagination, our intent, our motivation, our attitude. If we fail here, we may end up with an evil heart.