Why is Jonas's first memory of pain (when he loses control of the sled) an important key event in The Giver.
At the Ceremony of Twelve, the Chief Elder advises Jonas that his training will involve "Physical pain...of a magnitude that none of us here can comprehend because it is beyond our experience."... For, in the anathesized utopia in which Jonas dwells, pain has been eliminated. But, in Chapter 14 the Giver transmits the memory of a second sled ride that crashes, leaving Jonas with the sensation of a broken leg and the nausea that accompanies such a fracture. Afterwards, Jonas asks why he must feel this pain; the Giver explains that wisdom is acquired through such pain. However, in the event to make everyone's life safer and more content and unburdened of such memories, pain has been eliminated.
However, it is often from painful situations that people learn. Moreover, history is very much the recordings of man's pain. Thus, from his experience of pain, Jonas comes to realize that people only make wise decisions when they have some experience upon which to base a decision. This is why the Committee blindly takes the advice of the giver: not having felt any pain, not knowing any history, the members of the Committee have no basis for any decisions on its own.
His breaking of his leg is paramount because after having suffered pain, Jonas awakens to the fact that people are "satisfied with their lives which had none of the vibrancy his own was taking on." Now, his perception of his world begins to change: Jonas awakens to the stultifying world of which he has been a part, and finds it undesirable.