I think Adam was a very, very obedient soul. He may never have fallen into mortality on his own due to his obedience. However, it was necessary for Adam and Eve to fall. God knew from before the time he placed Adam and Eve into the Garden that they would fall and went ahead anyway and placed the two innocent beings in the garden. We know that God knew the fall would happen because he called Jesus to be a savior before the foundation of this world.
1 Peter 1:18-20
18 Forasmuch as ye know that ye were not redeemed with corruptible things, as silver and gold, from your vain conversation received by tradition from your fathers;
19 But with the precious blood of Christ, as of a lamb without blemish and without spot:
20 Who verily was foreordained before the foundation of the world, but was manifest in these last times for you,
Eve was beguiled by the devil through a series of lies mingled with truth and a desire to become like her father in heaven.
Here is the temptation Eve received:
Genesis 3:1-6
1 Now the serpent was more subtil than any beast of the field which the Lord God had made. And he said unto the woman, Yea, hath God said, Ye shall not eat of every tree of the garden?
2 And the woman said unto the serpent, We may eat of the fruit of the trees of the garden:
3 But of the fruit of the tree which is in the midst of the garden, God hath said, Ye shall not eat of it, neither shall ye touch it, lest ye die.
4 And the serpent said unto the woman, Ye shall not surely die:
5 For God doth know that in the day ye eat thereof, then your eyes shall be opened, and ye shall be as gods, knowing good and evil.
6 And when the woman saw that the tree was good for food, and that it was pleasant to the eyes, and a tree to be desired to make one wise, she took of the fruit thereof, and did eat, and gave also unto her husband with her; and he did eat.
The lie that Satan told Eve was that she would never die. But it was true that they would become more like God and receive wisdom. For God said,
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:
23 Therefore the Lord God sent him forth from the garden of Eden, to till the ground from whence he was taken.
So basically, God put 2 innocent beings in a garden, and he put the tree of knowledge of good and evil in the garden also. He put Satan there to tempt them and knew that Eve would succumb to the temptation. It was basically the plan of God to have his children come to earth to go through the mortal experience. This mortal experience is essential for man to learn how to choose good over evil of his own free will, to gain experience by receiving a body, and to walk by faith outside of being in God's presence or having a memory of God from their pre-mortal experience. All this experience helps man to gain wisdom and knowledge to progress and become more like the Father.
So even though Eve succumbed to Satan's temptation, God used Satan to further God's plan to bring his children into mortality to learn and progress to become more like God. God also used the innocence of Eve and her desire to be like the Father to bring about his purposes. Adam could not fulfill the commandment to multiply and replenish the earth without staying with Eve and therefore he partook also of the forbidden fruit and the rest is history.
2 Nephi 2:24-25
24 But behold, all things have been done in the wisdom of him who knoweth all things.
25 Adam fell that men might be; and men are, that they might have joy.
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