“[Adam and Eve] heard the sound of the Lord God walking in the garden at the time of the evening breeze, and the man and his wife hid themselves from the presence of the Lord God among the trees of the garden.” (Genesis 3:8, NRSV)
In the Garden of Eden, God provided for every possible need that Adam and Eve might experience. God created them to lavish on them the love that Trinity shares with each other. The Garden perfection was one such expression of love. For Adam and Eve to bear God's image, they, too, needed to be able to love. Without free will, love is impossible. So, God had to present Adam and Eve with a choice that would allow them to exercise free will.
There was a unique tree in the middle of the Garden. It was the Tree of the Knowledge of Good and Evil. Eating the fruit of this tree could give whoever ate it the ability to decide between that which is good and that which is not good—a decision best left in God's hands. In essence, if Adam and Eve chose to eat of that fruit, they would assume the role of judge, thereby usurping God's sole authority to be the perfect Judge.
Another way to put it is that they would begin to think they knew more than God about what is good and what is not good. With some prompting from a snake, they exercised their free will, and they chose to eat from that tree.
Immediately, they saw each other in a new way and felt shame in their seeing. They covered their nakedness with fig leaves (which must have been very uncomfortable due to the coarse texture), and they hid in fear that God was going to find out what they had done. As God took his usual walk in the Garden in the evening breeze, the absence of Adam and Eve, the objects of God's love, were missing from the scene. God called out, knowing full well what had happened, "Where are you?" to which Adam replied, "I heard the sound of you in the garden, and I was afraid, because I was naked; and I hid myself" (v. 10).
Much could be said here in explanation; however, I want to focus on one point. By declaring he was afraid of God, Adam judged God not to be good. If God is good, then why would Adam have to be frightened? After his declaration and God's question about whether Adam had eaten of the forbidden fruit, Adam began the blame game. And the rest is history. The pattern of declaring independence from God and then blaming others—like our partner, or the snake, or the devil—is set into the way we relate to God, to one another, and the world.
Sin is ultimately our decision to act independently of God and to exert our will and our way into any given situation. Sin is our attempt to be in charge and make decisions that are obviously in our best interest—all the while failing to recognize that we don't know as much as God and, therefore, our ability to see is limited. As a result, the choices we make, independent of God, may not be in our best interest. We fall short of God's glory in which we were created and live a life that is less than the life God designed us to live. Our sin is an offense to God only to the extent that God grieves for the deficit between the life we live and the life God had in mind for us. If we believe this to be true about God, then we have nothing to fear, for our God is most definitely good.