Why did Lois Lowry write the book The Giver?

Lois Lowry won the John Newbery Award for The Giver in 1994. In her acceptance speech, Lowry gave some reasons for writing the book.
She mentioned a specific interaction with her 90-year-old father in a nursing home. At the time, Lowry remembers that there were family pictures on the wall of her father's room. She also remembers that, on one particular occasion, he had pointed to a picture of Helen (her sister) on the wall. Helen had died from cancer at a very young age, so she was no longer with the family. What surprised Lowry, however, was her father's comments: he had pointed to Helen in recognition but could not remember what happened to his firstborn child.
Lowry remembered thinking that it is a human thing to try to forget pain. However, she wondered whether it was a safe thing to forget. Lowry's interaction with her father became an inspiration for The Giver, a novel in which people are deprived of the memories of suffering, grief, and pain.
During another interaction (this time with an audience member during her Newbery acceptance speech for Number The Stars), she was led to explain why the Holocaust story needs to be constantly revisited. Lowry's implication is that humans easily forget the evil of the past and are prone to repeat it. She relates that she herself was protected from the evil of "Elsewhere" when she was a child. Yet, she constantly sought answers to what existed beyond the horizon of her safe childhood experiences. Lowry gave this same curiosity to Jonas in The Giver.
Lowry mentions that it is tempting to live in a walled-in world where violence, poverty, and injustice technically does not exist. However, when we do that, we forget about others who are experiencing pain and injustice. To Lowry, the people in The Giver have lived in a sterile world for so long that they are in danger of losing the real emotions that make them human.
For more insights into why Lowry wrote The Giver, please refer to the link below.
http://newberryproject.blogspot.com/2013/03/the-giver-1994-medal.html

Lois Lowry first had the idea to create the world of The Giver when her father began suffering from memory loss in his old age.  She saw the suffering and emotional pain he went through when she reminded him that his other daughter had passed away many years before.  This gave Lowry the idea to create a fictional world in which there was no pain.  There were also no painful memories in such a world.  
Her father did not have control of his own memory in the later years of his life.  This made Lowry consider what life would be like if people did not have control of their memories.
In The Giver, members of the community do not have memories of pain or grief.  The Giver himself is the only person who holds these memories.  He protects the community by keeping these memories so they do not have to.  When the leaders in the community have a question about solving a problem, the Giver counsels them based on his memories.