The simple answer is that the serpent (the devil) persuaded Adam that eating the apple would be to his benefit. Adam would “become like God” if he ate it. Sadly, Adam believed this lie and chose to disobey God who had told him not to eat from the Tree of the Knowledge of Good and Evil. This was the first sin and resulted in humanity falling from what we call Original Innocence. But to give a more thorough explanation, let’s consider the entire story in a broader way.  
 
First of all, did Adam actually eat the apple? Perhaps, but perhaps not. It’s important to understand that we should not read this story in a “literal” way. That doesn’t mean the story is not true. A good explanation of this comes from Cardinal Ratzinger (who later became Pope Benedict XVI) in his book “In the Beginning …: A Catholic Understanding of the Story of Creation and the Fall” (Wm. B. Eerdmans Publishing). In that book, Cardinal Ratzinger stated, “It has become clear that the biblical creation narratives represent another way of speaking about reality than that with which we are familiar from physics and biology.” He went on to explain that these stories “represent truth in the way that symbols do — just as, for example, a Gothic window gives us a deep insight into reality, thanks to the effects of light that it produces and to the figures that it portrays.”  
 
In other words, the story of Adam and Eve is true, but not necessarily in the way a science book is factual. A science book presents truth in a literal way, explaining the physical truths as they are in this world. The story of Adam and Eve presents truth in the form of a symbolic narrative. Whether or not Adam actually ate the apple in the literal way explained in that symbolic narrative is not important. What is important is the truth that the narrative presents.  
 
So what is the truth it presents? It presents us with the fact that our first parents turned from God in disobedience. The consequence was a fall from the original state of innocence and union with God for which God had made them. Therefore, the answer remains that Adam “ate the fruit” for selfish motivations because he believed the lie that disobeying God was better for him than obeying God. He was selfish and failed to trust the truth of God.

Romans 3, verse 10 says, “…as it is written: ‘None is righteous, no, not one.’” Yet, James 5:16 says that the prayer of a righteous man availeth much. If absolutely no one is righteous, then who is James talking about? Luke chapter 1 says that Elizabeth and Zechariah were righteous before God. If absolutely no one is righteous, then how can that be? Is Scripture contradicting itself? No, the folks who interpret Romans as saying absolutely, without exception, no one is righteous, are misinterpreting that passage. They are failing to realize that the key to understanding Romans 3:10 is the phrase, “it is written.”  
 
Here in Romans, Paul is quoting from the Old Testament, Psalm 14 to be exact. In Psalm 14 it says, “The fool says in his heart, ‘There is no God. They are corrupt…there is none that does good.’” But then that same psalm goes on to talk about the “righteous.” Well, if none has done good, who are these righteous the psalm is talking about? Obviously, when the psalmist says that none has done good, he is talking about the fools who say there is no God. He is not talking about absolutely everyone.  
 
Just so Paul when he quotes from this psalm. Paul is not saying absolutely no one is righteous, if he was, then how do you explain all the Old and New Testament passages that refer to the righteous? In Romans 3:11 it says that no one seeks for God. Does that mean that absolutely no one is seeking God? No, to interpret it that way would be ludicrous!  
 
Just so verse 23 which says that “all have sinned”. Babies haven’t sinned, have they? Little children haven’t sinned, have they? No! This is not an absolute. There are exceptions. What about John the Baptist? Did he sin? Scripture says that he was filled with the Holy Spirit even from his mother’s womb. Can someone who is filled with the Holy Spirit his entire life ever sin? It’s something to think about.  
 
So, it is perfectly legitimate to say that these passages from Romans, when interpreted in context, in no way conflict with the Church’s teaching on Mary being without sin.