You have to read the entire second chapter of 2nd Nephi to understand. Moses 5:1-12 also helps.
Similar to the Asian doctrine of the Yin-Yang, there must be an opposition in all things. There is darkness to contrast with light, coldness to contrast with warmth, sickness to contrast with health, down to contrast with up, sorrow to contrast with happiness, evil to contrast with goodness.
We've been sent to earth to experience a world different from Heaven. In the Garden, Adam and Eve lived in a state of innocence, knowing neither good nor evil, neither happiness nor sorrow. God told them that if they chose to break the limits of the Garden, taking on a knowledge of evil and sorrow, then the world would fall from His glory. But they would finally be able to fulfill His commandment to produce children. Thus, Adam and Eve opened the doors to the school of mortality, and our graduation is made possible by Jesus Christ.
God's plan of happiness has three pillars: Creation, Fall, Atonement. Since before the foundation of the world, Jesus had been preordained to ransom mankind from the Fall. We will be resurrected with the knowledge we've gained from this mortal experience, our spirits molded in the refiner's fire of hardship. It is like how when planning a military operation behind enemy lines, you plan both infiltration (the Fall) and extraction (the Atonement). Satan didn't understand this when he urged Eve to break the Garden's limits.