Why Did God Limit Our Days to 43,830?

     I've read through the Old Testament several times. In fact, I'm one of the few people I know that's my age that doesn't almost always skip over it and go straight to the "good stuff." Several times I've noticed that God is so fed up with humans and their shenanigans that He decides to tighten up the reins on their mortality. The Census Bureau projects that the average lifespan of an American born in 2010 will be 78.3 years. Since 1970, that number has increased by nearly 8. As we make leaps in medicine, technology, and our understanding of total nutrition, we can see marked increases in our longevity. The Bible tells us, however, that our years on Earth are paltry when compared to those during the infancy of the world. Methuselah lived to the ripe old age of 969. Heck, he was 187 before he had his first son, Lamech. That leaves us all with a very obvious question: what happened? Why did God choose to so drastically shorten the lifespan of humankind?
     We are told of God's decision to cut our years short in Genesis 6:3:"Then the LORD said, “My Spirit shall not abide in man forever, for he is flesh: his days shall be 120 years.”
     Interestingly, there is some debate as to what God really meant by these words. Some scholars seem to think that God was actually letting mankind know how long it was going to be until He unleashed His wrath on them by flooding the Earth. The theory is that He was basically saying, "Hey, guys. None of y'all are going to live more than 120 more years. Not unless you're lucky enough to be on this huge boat I've got in mind."
     From what I can gather, there seems to be a pretty good argument for refuting this theory: we are told that Noah was 500 when God made the decision to limit man's days. We are also informed that Noah was 600 years old when the Earth was covered in water: "Noah was six hundred years old when the flood of waters came upon the earth" (Genesis 7:6 ESV). Since this gives us only 100 years to work with and not 120, it seems pretty plausible that God was referring to man's actual lifespan.
     It seems to take a little while for the limitations to take effect. After the flood, the patriarchs of Genesis slowly begin to live shorter and shorter lives. Jacob, the father of what would come to be known as Israel, is the last person that we are told lives to be 120:

     I think part of the reasoning for shortening the time of each person on the Earth was pretty simple. Man had shown God that he was capable of much wrongdoing over the course of 800 or so years; if you decrease the amount of time he's on the Earth, you decrease how much time he has to do bad things. Murder, while perhaps not being "rampant" yet, had already been committed at least twice that we are told of, once by Cain and once by Lamech. Lamech's justification might even have been weaker when compared to Cain's jealousy of Abel and the favor Abel's sacrifice has found in God's eyes: " Lamech said to his wives:
 “Adah and Zillah, hear my voice;
  you wives of Lamech, listen to what I say:
 I have killed a man for wounding me,
  a young man for striking me"
(Genesis 4:23 ESV).
A man hit Lamech, so Lamech kills him. Done. God says that one basic theme was running through the heart of nearly everyone on Earth: evil. The Father is apparently so grieved that He says He is sorry He ever made man in the first place.
     It is very interesting to note that this part of the Bible was written down millenia ago; science was virtually non-existent then, yet take a look at the oldest recorded people. With the possibility of only one exception, Jeanne Calment (who died at age 122), the longest people manage to live only hovers just shy of 120. If nothing else about God's pronouncement of limiting our days piques your interest, that should.

http://en.wikipedia.org/wiki/Oldest_people
http://www.biblestudy.org/maps/life-span-of-bible-patriarchs-before-after-the-flood.html
http://www.totalhormonegenetherapy.com/downloads/OurMission.pdf
http://www.census.gov/compendia/statab/2012/tables/12s0104.pdf