Gen 2:16,17 "And the LORD God commanded the man, saying, Of every tree of the garden thou mayest freely eat: But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die."
Romans 6:23 says, "For the wages of sin is death..." God barred the way so man would/could not become an eternal sinner. Sin would have had no end to it.
However, God had already made provision for man's salvation. The last part of Romans 6:23 says, "...but the gift of God is eternal life through Jesus Christ our Lord."
Praise the Lord for His gift 

Very good question. Look at [Genesis 3:22], "And the LORD God said, Behold, the man is become as one of us, to know good and evil: and now, lest he put forth his hand, and take also of the tree of life, and eat, and live for ever:". The "us" mentioned is God referring to Himself and all the heavenly host (angels). When Adam disobeyed God (sinned), there wasn't just what God said to do anymore but now there was what seemed and felt good to do as well. Now another choice was opened up to man that God hated because He wants to be the only choice and He is the only choice for any that want to dwell with Him forever. This situation was repeated in a different way in Genesis 11 with the tower of Babel and in Numbers 14:40-45. People are constantly "creating" other choices to get to eternal life but God has maintained that there is only one. He would not and will not allow man to continue in sin and still receive eternal life. That's why the tree of life was blocked off, the languages confused at the tower of Babel and the Hebrews killed before entering into the promised land. Jesus confirms this in [John 10:1], "Verily, verily, I say unto you, He that entereth not by the door into the sheepfold, but climbeth up some other way, the same is a thief and a robber."