Genesis

I had a quick question about a verse from the reading for a few days ago. In Genesis 3:22 talks about not allowing man to eat from the tree of life. My question is, why not? Is it because man is sinful, so God doesn't want him to live in that sin forever? Here are the verses:
"22 And the LORD God said, "The man has now become like one of us, knowing good and evil. He must not be allowed to reach out his hand and take also from the tree of life and eat, and live forever." 23 So the LORD God banished him from the Garden of Eden to work the ground from which he had been taken. 24 After he drove the man out, he placed on the east side of the Garden of Eden cherubim and a flaming sword flashing back and forth to guard the way to the tree of life."

Also yesterday I read a simple, but powerful verse that got me thinking:
Genesis 6:22- "Noah did everything just as God commanded him."
Wow! The guy is over 500 years old and God wants him to build a gigantic boat, gather 2 of every single animal, and trust that everything will be okay. And the surprising thing to me is Noah's response. The Bible doesn't say "Noah thought God was crazy, but did it anyway" or "Noah doubtfully obeyed what God told him". It doesn't say anything about Noah questioning what God was doing. It just says "Noah did everything just as God commanded him." He didn't complain, question, or doubt, he just did!