When the human race began to increase, with more and more daughters being born,
the sons of God noticed that the daughters of men were beautiful. They looked them over and picked out wives for themselves.
Then God said, "I'm not going to breathe life into men and women endlessly. Eventually they're going to die; from now on they can expect a life span of 120 years."
This was back in the days (and also later) when there were giants in the land. The giants came from the union of the sons of God and the daughters of men. These were the mighty men of ancient lore, the famous ones. Noah and His Sons
God saw that human evil was out of control. People thought evil, imagined evil - evil, evil, evil from morning to night.
God was sorry that he had made the human race in the first place; it broke his heart.
God said, "I'll get rid of my ruined creation, make a clean sweep: people, animals, snakes and bugs, birds - the works. I'm sorry I made them."
But Noah was different. God liked what he saw in Noah.