It's amazing to think that in the time before the flood, during the age of the Patriarchs, the average lifespan was about 900 years. After the flood, the average age leveled off to around 110. In fact, there were several reductions in a Man's human lifespan. Reduction #1 was after Adam sinned. The average lifespan was around 900 years. Reduction #2 was after the flood, when God reduced lifespan to around 120 years. Reduction #3 was after Moses. Psalm 90:10. We read that the average lifespan has been reduced down to between 70 and 80 years.
Consider this: If Genesis 6:3 did not exist in the Bible, it is still evident that man's lifespan was shortened from: being infinite, to about 900 years, to about 120 years, to about 70 to 80 years. We were meant to live forever.