When the human population began to grow rapidly on the earth,
the sons of God saw the beautiful women of the human race and took any they wanted as their wives.
Then the LORD said, "My Spirit will not put up with humans for such a long time, for they are only mortal flesh. In the future, they will live no more than 120 years."
In those days, and even afterward, giants lived on the earth, for whenever the sons of God had intercourse with human women, they gave birth to children who became the heroes mentioned in legends of old.
References for Genesis 6:4
Now the LORD observed the extent of the people's wickedness, and he saw that all their thoughts were consistently and totally evil.
So the LORD was sorry he had ever made them. It broke his heart.