What would be an answer you would take as a direct answer to your "Why did Adam sin?"
Suppose I were to answer, "Because Adam wanted to do what he knew he should not do."
Assuming you would then ask, "Well why did Adam want to do what he did?"
My answer would be something along the lines of...
"Adam was aware of God’s commandment at the moment he ate the forbidden fruit, Adam possessed the capacity and power to obey God’s preceptive will, for reasons sufficient to him (his self-determined inclinations at the moment) Adam wanted to eat the fruit, and Adam was not forced to eat the fruit (no violence done to his will). Thus, because Adam acted knowingly, willingly, with freedom of spontaneity, for reasons that were sufficient to him, with no violence done to his will, Adam was a free moral agent in his act of sin. In fact, given that sin begins in the mind’s choosing and not in the act, it can be said that Adam sinned before he took the first bite of the apple."
Perhaps you would then press me further asking, "Well, why did Adam want to eat of the forbidden fruit?"
I might say,
"Eve had eaten, and handed Adam the fruit with one bite taken. Neglecting his duty to his wife as her spiritual protector, being beguiled by the Temper's argument—"hath God said?"—an argument Adam considered valid through an internal defect in his understanding of what God had actually commanded, Adam acted on his inward inclinations.
"So in the final analysis, it seems to me the why comes down to Adam's mutability, he being made able to act contrarily to that which he ought to do. Blame attaches to actions, and actions are characterized by intentions. Adam was able to sin and able not to sin, but he did not yet have a sin nature. Adam's nature was not neutral. There was nothing in Adam's nature that in any way prompted him to sin. Yet Adam was not yet glorified and Adam had the capability of sinning (and did)."
Is any of the above within the realm of what you are looking for as an answer?
When you say, "for reasons sufficient to him [Adam]," you've already acknowledged the mystery I'm arguing for. If a man commits a crime, and we say that his motive consisted of reasons that were "sufficient to him," have we solved the mystery of the crime? No -- we've actually affirmed it. And in the case of Adam the mystery is far deeper, because Adam had no pre-existing set of sinful inclinations to use as a motive. The fact that Adam had the power to sin doesn't mean that he didn't also have the power not to sin. He had the power to do either. This power of his, the power of mutability, explains only the "how" of the fall, not the "why."