I believe that Lowry is attempting to emphasize to her readers the importance of dealing with issues in life and the importance of memories. Events shape our lives, both the good and the bad. In order to experience a balanced existence, one needs to have experienced both hurt and happiness. In The Giver, the idea of a perfect world is that no one should know pain, lonliness, sickness, or any feelings that resemble anything less than an idyllic life. Thinking about my own life, for example, I can't imagine that I could possibly appreciate the happy events in my life--family, friends, loved ones, births--without the opposite--loneliness, loss, and death--to compare these events to. Many people bemoan sad or tragic events in their lives, and I believe that Lowry's point in this book is to allow the reader a glimpse into a perfect or idyllic world to see that many times appearances can be misleading, and not everything is as it seems. In other words, hard times are necessary in order for us to appreciate good times. I believe that this is her reason for for sending Jonas on his quest at the end of the novel, he is in search of a real life, with both the good and the bad.
I answered a similar question on the Discussion Boards a few days ago, the link is listed below.
To add to it I think Lowry had several messages she wished to impart on her audience in writing this book. Some themes, other than the one I wrote about, that I think Lowry developed well were these of Death and society versus the individual. Jonas' father works to "release" babies. Jonas has always kind of admired his father's work thinking that it had something to do with easing pain and helping people. In the final chapters of the book Jonas learns that to be "released" mean to be euthanized. When this is revealed to Jonas he is horrified because he has learned to truly value life where others in his society have not. This message that Lowry sends I think is a warning to people not to allow a society total control over the individual because it becomes difficult to regain freedom that is lost on such a grand scale. No one understands Jonas' strife over "releasing". No one understands the true value of life. These rituals have become cultural norms. I think she wants to tell her readers that just because a nice soothing name is put on something like "release" doesn't mean there aren't horrific implications like Jonas learned in the novel.