Genesis 6:5 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.

Humans always want to do their own thing. They are not willing to believe the Bible or what God has to say. Humanity imagines themselves as making their own decisions, indirectly being a god. They would never admit that, but the public, people, and the individual man want to decide what is right and wrong.

God looked at the man. He saw that man was and will always be a failure. Man will always do wrong. Man is a sinner. He may think he is doing good, but only because he is his judge or public opinion, not what God says!

Humans, naturally, do not want to know what God says. They do not want to live up to a standard they didn't set. Their thoughts are of themselves. What they think and what they feel are their greatest motivations. 

Man wants to decide what is right! That way, he can feel good about it and himself. 

Their thinking evil continually doesn't mean that they always do horrible things. It means that God is far from their thoughts. It means that they have rebelled against their creator God. It means they want to do whatever they want, not what God wants.