What caused Adam and Chava to sin; to do the wrong thing; to make the wrong choice: What was it in their human being, thats being called Tov me'od (Bereshit 1:31), that drove them into eating from the fruit of the tree of knowledge of good and evil while it was forbidden them to do so?
I heard an explanation, which was attributed to R' Moshe Shapiro, that basically said the following (sorry I don't have his sources, I heard it second-hand):
Adam was given a supremely spiritual and holy state of being, to the point that the angels wanted to praise him. Yet G-d told him to do a very small task - don't eat from this tree, for a few hours. He was given incredible spiritual potential, and instructed not to do anything with it except to not mess things up!
The challenge for Adam was to forgo participating in perfecting the world, to submit his own desire to activate his spiritual potential for the sake of doing what Hashem said. It was a case of doing what G-d said despite what you think would be best, with the added element of the humility required to not do what he knew he could do.
To put it differently, Adam wanted to "mess the world up" in order to use his great potential to fix it - to show that he could bring the world to its fulfillment through his own spiritual accomplishments. The problem was that G-d said not to.
So the challenge was having the humility to not exert his own abilities, and not to follow his own idea of what was right.
The way the Yaavetz explains the Avos Drav Nosson, Adam made a safeguard for God's commandment. God said don't eat from the tree of knowledge, and Adam made a safeguard and told Chava not to touch the tree as well. However, he never told her that this was his addition so the plan backfired. The snake pushed Chava onto the tree and said you didn't die from touching it, you won't die from eating it. And so she ate from it.
It's interesting to note that the Avos Drav Nosson puts this on a list of safeguard enactments which were instituted over time as a way to bring the message of the mishna in Avos home: asei syag laTorah, make a safeguard for the Torah. What's interesting is this seemingly ignores the fact that this safeguard backfired! It might be though that this is the very lesson being taught. Some safeguards will backfire. But they are still necessary, and better than the alternative of trying to keep the commandments without them.